35 Min Read

Blood Cells: Definition, Types, Structure and Functions

Photo of author

Sourav Bio

What are Blood Cells? – Blood Cells Definitions

Blood is a crucial fluid that travels throughout the bodies of many animals, and its biological components are called blood cells. The functions of blood cells include delivering oxygen, nutrients, hormones, and other chemicals to where they are needed and eliminating waste products and carbon dioxide from those areas.

  • The red blood cells were first seen under a microscope by the Dutch naturalist Jan Swammerdam in 1658, and the first picture of red corpuscles was made by the Dutch microscopist Antoni van Leeuwenhoek in 1695.
  • Until the discovery of platelets by French physician Alfred Donné in 1842, no further types of blood cells had been identified. The next year saw the simultaneous discovery of leukocytes by French professor of medicine Gabriel Andral and British physician William Addison.
  • They shared a view that both red and white blood cells were impacted by sickness. Hematology as a medical speciality was founded on these findings.
  • Blood cell morphology was largely unknown until Paul Ehrlich published his method for staining blood films and for differential blood cell counting in 1879. This was despite the availability of chemicals for staining tissues and cells.
  • Blood cells are divided into three major categories: red blood cells (also called erythrocytes), white blood cells (sometimes called leukocytes), and platelets (also known as thrombocytes).
  • Transporting oxygen from the lungs to the tissues and eliminating carbon dioxide from the tissues are two of the main functions of red blood cells. White blood cells are a key component of the immune system, which works to ward off illness and infection. Involvement of platelets in blood clotting aids in stopping excessive bleeding.
  • Hematology, the study of blood cells, has been around since the time of traditional medicine practises like bloodletting. Scientists weren’t able to closely examine blood cells until the microscope was invented in the 17th century.
  • Improvements in the treatment of blood problems and diseases have resulted from technological and scholarly developments made possible by studying blood cells. Several diseases and disorders, from anaemia to cancer, are diagnosed and monitored with the help of blood tests because of their ability to quantify the amount and properties of various blood cells.

Characteristics of Blood cells

  • Blood cells are biological components of blood, which is a fluid that travels through the body.
  • There are three primary types of blood cells: red blood cells, white blood cells, and platelets.
  • Red blood cells (RBCs) are tiny, disk-shaped cells that don’t have a nucleus or any other organelles.
  • While both red and white blood cells have a nucleus and other organelles, white blood cells are much larger.
  • Platelets are small cell fragments that lack a nucleus and are crucial in blood coagulation.
  • The spongy tissue known as bone marrow is located inside the bones and is responsible for producing blood cells.
  • From a few days to a few months, blood cells live and then die off, to be replaced by new ones.
  • Blood cells can be impacted by a wide range of diseases and situations, including infections, genetic abnormalities, and cancer.
  • A common diagnostic tool in medicine is a blood test, which can reveal a lot about a patient’s health just by looking at their blood cells.
  • Blood transfusions, which include the transfer of blood or blood products from one person to another, are occasionally used to treat medical diseases that affect blood cells.
  • Age, sex, and health all play a role in how many and what kinds of blood cells circulate through the body.
  • The cells that make up blood are always on the move, travelling through the body’s vascular system to deliver oxygen, nutrition, and other vital materials.
  • Hormones, diet, and physical activity have all been linked to changes in blood cell synthesis and function.
  • Microscopy, flow cytometry, and other laboratory methods allow for the visualisation and study of blood cells.
  • Inflammation, wound healing, and immunological response are just a few of the activities that benefit from the ability of blood cells to communicate and collaborate with one another and with other cell types and tissues.
  • Many diseases and symptoms can originate from blood cell abnormalities like low red blood cell count or aberrant white blood cell function.
  • The environment can have an effect on blood cells through things like poisons, radiation, and diseases.
  • Because they contain DNA and other genetic material, blood cells can also be used for research and diagnostics.
  • Leukemia, lymphoma, and myeloma are just a few examples of the many diseases that impact blood cell development and function.
  • The discovery of new therapies for a wide range of blood illnesses and diseases is largely attributable to the progress made in the study and study of blood cells.

Contents of Blood Cells

Contents of Blood Cells
Contents of Blood Cells
  • After being left for half an hour, blood in a test tube will separate into three distinct layers, with the denser components falling to the bottom of the tube and the fluid remaining at the top.
  • About 60 percent of blood is made up of plasma, a straw-colored fluid that makes up the top layer. White blood cells (WBCs) and platelets make up the middle white layer, while the red blood cells make up the bottom red layer (RBCs). About 40% of blood is made up of these two layers of cells.
  • Although it is mostly water, plasma contains a number of other substances that are vital to the body, including proteins (albumin, clotting factors, antibodies, enzymes, and hormones), sugars (glucose), and fat particles.
  • Bone marrow is the source of every single blood cell. Red blood cells (RBCs), white blood cells (WBCs), and platelets (PLTs) all develop from an initial stem cell. Lymphocytes, monocytes, and granulocytes are the three main types of white blood cells, and there are also three main types of granulocytes (neutrophils, eosinophils, and basophils).
  • By centrifuging a blood sample, its constituents can be further isolated. Denser elements sink due to the force of the spinning, and subsequent processing allows for the isolation of a certain protein or blood cell type. Using this technique, plasma can be extracted to treat immunological deficiencies with antibodies and bleeding problems with clotting factors. Red blood cells (RBCs) can also be extracted for use in transfusions.
Composition of whole blood.
Composition of whole blood. | Image Source: https://basicmedicalkey.com/blood-3/

Types of Blood cells

There are three main types of blood cells:

  1. Red blood cells (RBCs): RBCs, or erythrocytes, are the body’s most common type of blood cell. They are responsible for delivering oxygen to the body’s tissues from the lungs and returning carbon dioxide to the lungs from the rest of the body. Red blood cells (RBCs) are coloured red because of a protein called haemoglobin that binds to oxygen.
  2. White blood cells (WBCs): White blood cells (WBCs), sometimes called leukocytes, are an integral element of the immune system that works to ward off illness and infection. Lymphocytes, neutrophils, monocytes, eosinophils, and basophils are just few of the white blood cell kinds. Several types of white blood cells perform specialised tasks include making antibodies, fighting off invaders, and controlling inflammation.
  3. Platelets: When blood clots, it’s thanks in part to platelets, also called thrombocytes, which are pieces of larger cells. Platelets immediately clump together at the site of a blood vessel injury to form a plug and halt bleeding. In addition, they secrete substances that stimulate blood clotting, which ultimately leads to the creation of a firm clot that effectively closes the incision.

Red blood cells (RBC) or Erythrocytes

Red blood cells, also known as erythrocytes, are the most numerous cells in the bloodstream, accounting for about 40-45% of the total blood volume. They are small, disc-shaped cells that lack a nucleus and most other organelles.

  • Hemoglobin is a protein found inside RBCs that binds to oxygen and carbon dioxide. Red blood cells (RBCs) are responsible for transporting oxygen from the lungs to the rest of the body and carbon dioxide from the rest of the body back to the lungs to be expelled.
  • Red blood cells (RBCs) are created in the spongy tissue located inside of bones called bone marrow. The kidneys release a hormone called erythropoietin in response to low oxygen levels; this hormone regulates the synthesis and maturation of RBCs.
  • After around 120 days, RBCs are withdrawn from circulation and metabolised by the liver and spleen. Red blood cells (RBCs) are constantly being renewed by the body.
  • Blood tests such as the complete blood count (CBC) and the hematocrit can be used to determine the quantity and quality of RBCs in the blood.
  • Anemia, sickle cell disease, and thalassemia are only few of the disorders that can result from a lack of RBCs, an aberrant RBC size or shape, or an abnormal synthesis of haemoglobin.
  • Environmental variables, such as toxin or radiation exposure, and nutritional deficiencies, such as iron or vitamin B12 insufficiency, can have an impact on RBCs.
  • Conditions like severe anaemia and bleeding disorders can necessitate a transfusion of RBCs to address the resulting anaemia or blood loss.
  • New therapies, including as gene therapy for sickle cell disease, have emerged thanks to advancements in RBC research and technology.
Normal human erythrocytes.
Normal human erythrocytes. | Image Source: https://basicmedicalkey.com/blood-3/

Structure of Erythrocytes or Red blood cells (RBC)

Known by their scientific name, erythrocytes, red blood cells are responsible for transporting oxygen around the body. Some prominent characteristics of RBC structure are as follows:

  • Red blood cells (RBCs) have a biconcave disc form, indicating that both sides are concave. As a result, they have a large surface area in relation to their volume, making petrol exchange relatively easy.
  • The diameter of an RBC is only about 7-8 micrometres, making it one of the smallest cells in the body. For this reason, they are able to navigate the capillaries, the body’s tiniest blood arteries. (They have a diameter of around 7.5  μm, a thickness of 2.6  μm around their periphery, and a thickness of about 0.75  μm in the middle.)
  • Red blood cells (RBCs) are devoid of a nucleus and virtually all other organelles. Because of this, they are able to store more haemoglobin, the protein that binds oxygen and carbon dioxide, in their bodies.
  • Red blood cells (RBCs) have specialised proteins including band 3 protein and glycophorin A in their lipid bilayer cell membrane. These proteins aid the RBC in staying both flexible and shaped so that it can pass through tiny blood vessels.
  • Hemoglobin, found only in RBCs, accounts for nearly a third of these cells’ total mass. The iron-containing heme molecule is attached to each of the four protein subunits that make up haemoglobin. Hemoglobin’s ability to bind oxygen and carbon dioxide is a result of the iron in heme.
  • Blood cells include a variety of enzymes and proteins, including carbonic anhydrase, which controls blood pH, and spectrin, which reinforces the cell membrane.
  • The flexibility of erythrocytes allows them to conform to the narrow confines and asymmetrical architecture of the blood arteries.
  • Erythrocyte plasmalemma is the most well-studied cellular membrane because it is so abundantly present in the body. There are around 10% carbohydrates, 40% lipids, and 50% proteins in the plasmalemma.
  • The cytoskeleton is a network of proteins that helps keep the red blood cell’s phospholipid bilayer membrane in place.
  • The cytoskeleton, which is made up of proteins including spectrin, actin, band 3, protein 4.1, and ankyrin, provides structural stability and flexibility to the cell.
  • There are no organelles in an erythrocyte’s cytoplasm, but it is packed with haemoglobin, which carries oxygen and carbon dioxide around the body.
  • Red blood cells (RBCs) only remain in circulation for about 120 days before being removed and broken down in the liver and spleen. The bone marrow is constantly replenishing its supply of red blood cells.
  • Many diseases and ailments can be caused by alterations in the structure of RBCs, such as the sickle cell shape seen in sickle cell disease or the aberrant size and shape seen in other disorders.
  • On average, there are between 3.9 and 5.5 million erythrocytes per microliter (μL, or mm3) of blood in women and between 4.1 and 6.0 million erythrocytes per μL, or mm3, in men.

Functions of Erythrocytes or Red blood cells (RBC)

Erythrocytes, often known as red blood cells, are responsible for transporting both oxygen and carbon dioxide throughout the body. Here are some of erythrocytes’ most important roles:

  • Oxygen transport: Hemoglobin is a protein found in erythrocytes that binds to oxygen in the lungs and then releases it in the body’s tissues; this is how oxygen is transported around the body. In this way, oxygen may be brought into the cells where it is required for respiration.
  • Carbon dioxide transport: Erythrocytes carry carbon dioxide from the tissues to the lungs, where it can be expelled as a waste product of cellular respiration.
  • Acid-base balance: Erythrocytes have carbonic anhydrase, an enzyme that helps maintain the body’s acid-base balance by converting carbon dioxide and water into carbonic acid. This aids in controlling blood acidity and alkalinity.
  • Blood viscosity: The high concentration of erythrocytes gives blood its thick, viscous consistency. This aids in preserving healthy levels of blood pressure and circulation.
  • Iron storage: As heme molecules, erythrocytes store a considerable amount of iron. In the process of destroying erythrocytes, this iron can be reused or stored for later use.
  • Immune function: Erythrocytes, by transporting antigens on their surface, can contribute to immune function. Antibodies produced in response to these antigens may prove useful in the battle against infection.
  • Nutrient transport: In addition to carrying oxygen, erythrocytes are capable of delivering glucose and amino acids to cells throughout the body.

In the grand scheme of things, erythrocytes are essential for equilibrium and general bodily function. Disorders including anaemia and sickle cell disease can develop from erythrocyte dysfunction or abnormalities in erythrocyte quantity.

RBC Summery

  • Red blood cells (RBC), also known as erythrocytes, are the functional component of blood that carries oxygen and nutrients across the body. These specialized cells are able to perform their crucial roles because of their distinct structure and makeup.
  • Erythrocytes play an important part in the study of disease processes across many organ systems.
  • The mature erythrocyte is biconcave, discoid, and anucleated.
  • The cell is able to perform its function because of the design’s flexibility and the increased surface area that ensures adequate gas exchange.
  • The cytoskeleton is a protein network that supports the phospholipid bilayer membrane that defines this special type of cell. Spectrin, actin, band 3 protein 4.1, and ankyrin make up the cytoskeleton, which gives cells their rigidity and flexibility.
  • Structures that are both sturdy and malleable are supported by the interactions between these compounds.
  • The average lifespan of a red blood cell is only 120 days. With such a limited amount of time, the circulatory system must transport oxygen from the lungs to the rest of the body’s tissues, where it can be used in metabolic processes like ATP synthesis, and then bring back the waste carbon dioxide to be exhaled.
  • When oxygen-starved blood reaches the lungs, it carries hemoglobin containing ferrous heme (Fe), which has a strong attraction to oxygen. As heme reaches deoxygenated tissues, the low pH and low partial pressure of oxygen lead it to lose its affinity for the oxygen and release it into the tissue.
  • Carbonic anhydrase then incorporates the CO2 into the cellular environment, where it reacts with H2O to produce bicarbonate and hydrogen. A large portion of the CO2 will be recycled to the lungs as bicarbonate and exhaled.
  • Changes in erythrocyte size, shape, and distribution may provide insight into a variety of diseases and pathologies seen in the clinic. For instance, acanthocytes may form during the process of eryptosis, the controlled and gradual degeneration and death of erythrocytes.
  • Tabulated cell death is observed in anemia and hypercalcemia. Red blood cell agglutination may suggest a hypercoagulable state, while microcytosis, or abnormally small red blood cells, is linked to microcytic anemias including iron shortage and thalassemia.
  • The severity of the disease is also affected by the shape of the red blood cells. The severity of erythrocyte sickling, for instance, is linked to the amount of hereditary hemoglobin S. Blood smears can reveal the presence of these cells, which contribute to many of the disease’s clinical symptoms, such as vaso-occlusive crises and associated pain.
  • In addition, erythrocytes’ susceptibility to pro-inflammatory states and physiological alterations has been documented in the scientific literature.
  • Non-discoid or atypical erythrocyte morphologies are common in inflammatory disease conditions such systemic lupus erythematosus, but were reversible after inflammatory product chelation was administered.
  • The role of inflammation and oxidative stress on erythrocytes in the development of chronic inflammatory illness is further supported by this finding.
  • In addition, the analysis of a patient’s red blood cells can shed light on their overall health and the physiological processes taking place in their body.
  • Leukemias and other malignancies may involve the presence of nucleated red blood cells, which might be a sign of hemolysis, bleeding, or hypoxia.

White blood cells (WBC) or Leukocytes

White blood cells (WBC) are a heterogeneous collection of nucleated cells that circulate for at least a portion of their lifetime. Their typical blood concentration ranges from 4,000 to 10,000 per microliter. They serve a crucial role in phagocytosis and immunity, and consequently in the fight against infection.

  • Granulocytes, lymphocytes, and monocytes are the three divisions of white blood cells (WBC). Granulocytes derive their name from the granulation of their cytoplasm. Neutrophils (or polymorphonuclear granulocytes), eosinophils, and basophils are the three known types.
  • Myeloid cells are derived from a common pluripotent stem cell, CFU-S or CFU-GEMM. A more primitive stem cell gives rise to both lymphoid and myeloid precursor cells. Under the influence of poietins and microenvironmental factors, stem cells transform into mature blood elements through a series of intermediate steps.
  • CFU-S gives rise to burst forming unit (BFU)-E, which in turn gives rise to CFU-E and the erythroid series; CFU-MEGA gives rise to megakaryocytes; and CFU-GM gives rise to monocytes and granulocytes. Stem cells are morphologically similar to lymphocytes. They can be identified by their in vitro growth characteristics in semisolid media containing various growth factors.
  • In the evolution of the neutrophilic granulocyte, the myeloblast is the first cell that can be identified morphologically. As maturation advances, the myeloblast transforms into a promyelocyte and then a myelocyte. These stages of development are dominated by a proliferative compartment in which the number of cells increases geometrically.
  • The metamyelocyte cannot undergo further mitosis and instead transforms into a band. This cell is either released into circulation (3 to 5% of WBC) where it matures, or it enters a storage compartment in the bone marrow where it becomes a neutrophil and is released into circulation later.
  • Approximately half of the intravascular polymorphonuclear cells are circulating, maintaining a dynamic equilibrium with the other half, which are adherent to the vascular endothelium. In the WBC count, only neutrophils are taken into account. About 7 hours is the half-life of mature neutrophils in circulation. After 1 or 2 days, they pass irreversibly through the arterial endothelium and into the tissues, where they perish.
  • The primary function of neutrophilic granulocytes is bacterial phagocytosis. This is a complex, multistage process that involves engulfment of the organism, absorption into the cytoplasm, and fusion with a lysosome, where enzymes are released to destroy the bacterium while energy is created.
  • The development of eosinophils and basophils are comparable. After leaving the bone marrow, eosinophils immediately leave the intravascular compartment (where they account for up to 5% of WBC) and enter the tissues. They cannot reenter the bloodstream.
  • Eosinophils are abundant in the gastrointestinal tract, lungs, and skin. Unknown is the specific function of these complicated cells. They may contribute to the defense against multicellular parasites and the limitation of inflammation.
  • Around 1% to 2% of circulating leukocytes are basophils. Their physiologic function is likewise not precisely understood. In their granules, heparin and histamine are present. IgE is associated with their surface.
  • Together, macrophages and lymphocytes are referred to as mononuclear leukocytes. Both play crucial roles in cellular and humoral immune function. These cells are capable of leaving and reentering the bloodstream while preserving their function. They could reside in the tissues or lymph nodes.
  • The cells of the monocyte–macrophage system are derived from the CFU-GM in the bone marrow. They are not kept, but are promptly discharged into the bloodstream, where they make up 5% of WBC. In tissue, they transform into macrophages.
  • Monocyte–macrophages phagocytose bacteria and particulate matter, participate in the inflammatory response, and play a key role in the immune system, where they analyze antigenic material and “communicate” with T lymphocytes via a cell–cell interaction process. Monocytes are capable of secreting interleukin, which stimulates B and T cells. By releasing plasminogen activators, they contribute to fibrinolysis.
  • Lymphocytes are important immune cells for cellular and humoral immunity. In the blood, they account for 20 to 45 percent of WBC. They are members of the B (bursa or bone marrow) and T (thymus) immune systems. The two cells are morphologically identical.
  • The B system is responsible for antibody production. When a B cell is appropriately activated, it first multiplies and then transforms into a plasma cell, the effector arm of the immunological arch. Each B cell is only capable of producing a single species of antibody.
  • The T system governs the entire immune apparatus and constitutes the cellular immune system. Using monoclonal antibodies specific for several membrane antigens, it is possible to identify multiple subsets of T lymphocytes.
  • Helper T cells, for example, promote the function of B cells, while suppressor T cells block them. Some T cells are responsible for cell-mediated cytotoxicity, whereas natural killer (NK) lymphocytes are responsible for nonspecific cell lysis.
  • 15 to 25% of lymphocytes in the peripheral blood are B cells, while 40 to 75% are T cells.
White blood cells (WBC) or Leukocytes
White blood cells (WBC) or Leukocytes

1. Granulocytes

Granulocytes are a type of white blood cell characterized by the presence of granules in their cytoplasm that can be stained and visualized under a microscope.

  • The immune system relies heavily on granulocytes, a type of leukocyte.
  • Produced in the bone marrow, these cells are then discharged into the bloodstream.
  • There are three main types of granulocytes—neutrophils, eosinophils, and basophils—distinguished by the granules seen in their cytoplasm.
  • The most common type of granulocyte, neutrophils, are the body’s first line of defense against microorganisms.
  • Eosinophils play a role in the immune system’s defense against parasite infections and in the development of allergic reactions.
  • Basophils, the rarest form of granulocyte, have a role in allergic reactions by secreting histamine.
  • Granulocytes have enzymes and other chemicals in their granules that might be harmful to invaders.
  • Granulocytes have the ability to ingest and digest foreign particles or organisms through a process called phagocytosis.
  • A decrease or increase in granulocyte count may be an indicator of infection or an autoimmune illness.
  • As part of the inflammatory response, granulocytes can travel to the site of an injury or infection to aid in the fight against it.

a. Eosinophils

Eosinophils are a type of granulocyte or white blood cell that are characterized by the presence of large granules in their cytoplasm that can be stained with eosin, a red dye.

  • White blood cells (also known as leukocytes) called eosinophils play an important role in the body’s immunological system.
  • They are made in the bone marrow and then circulate throughout the body.
  • There is evidence that eosinophils contribute to both the immune response to parasite infections and allergy reactions.
  • They make up between 1% and 6% of the leukocytes in a person’s blood.
  • Depending on the situation, eosinophils can remain in the blood for several hours to a few days before moving on to other tissues.


  • The nucleus of an eosinophil is bilobed, and the cell’s cytoplasm is filled with enormous granules that are characteristic of eosinophils.
  • Invading infections can be combated with the use of enzymes, proteins, and poisonous chemicals stored in the granules.
  • Major basic protein (MBP) and eosinophil peroxidase (EPO) are two parasite-killing enzymes found in the granules.


  • Toxic granules released by eosinophils are a key component in the fight against parasitic infections.
  • Histamine and other inflammatory mediators are released by mast cells during allergic reactions, contributing to symptoms including itching and swelling.
  • Eosinophils aid in inflammation recovery by stimulating tissue regeneration and wound healing.
  • The presence of an abnormally high number of eosinophils in the blood may be an indicator of allergies, asthma, or a parasite infection.
  • Hodgkin’s lymphoma and certain kinds of leukemia have been linked to increased eosinophil levels.

b. Basophils

Basophils are a type of granulocyte or white blood cell that are characterized by the presence of large granules in their cytoplasm that can be stained with basic dyes.

  • The nucleus of a basophil is separated into two irregular lobes, and its cytoplasm contains granules (0.5  μm in diameter) that normally color purple when exposed to basic dyes.
  • When compared to granulocytes of other types, these granules are fewer in number, larger in size, and more irregular in shape.
  • Although basophils are also 12-15  μm in size, they account for less than 1% of circulating leukocytes and are thus difficult to detect in routine blood smears.
  • Granules include heparin and other sulfated GAGs, which gives them a striking basophilia.
  • Together with histamine and other inflammatory mediators including platelet-activating factor and eosinophil chemotactic factor and the enzyme phospholipase A, basophilic granules contain a lot of eosinophil chemotactic factor.
  • Basophils, which migrate into connective tissues and produce their granular components in response to certain antigens and allergens, supplement the functions of mast cells due to their surface receptors for immunoglobulin E (IgE).


  • The nucleus of a basophil is lobate, and the cell’s cytoplasm is filled with voluminous granules of basophilic material.
  • Histamine, heparin, and other inflammatory mediators are stored in the granules and can be released to fight off invaders.
  • Many types of immune cells can be activated by the cytokines and chemokines found in basophils.


  • By producing histamine and other inflammatory mediators, basophils play a critical role in allergic reactions, which can lead to itching, swelling, and redness.
  • As part of the body’s immune reaction, they help eliminate parasites by releasing granules of a poisonous substance.
  • The basophils’ ability to release cytokines and chemokines that recruit additional immune cells to the site of infection or injury is another way in which they influence the immune response.
  • Medical disorders include allergies, parasite infections, and even some types of leukemia can be indicated by abnormally high or low basophil numbers.
  • Certain autoimmune illnesses, such rheumatoid arthritis and systemic lupus erythematosus, have been linked to increased basophil numbers.

c. Neutrophils

Neutrophils are the most abundant type of granulocyte or white blood cell in the human body, characterized by the presence of small, neutral-staining granules in their cytoplasm.

  • Neutrophils are tiny scavenger cells that work quickly to clear damaged areas of the body of dead cells and debris left behind by invading microorganisms.
  • Nuclei of neutrophils are typically complex, with up to six lobes joined by a narrow nuclear extension; granules are lysosomes containing enzymes to breakdown ingested material.
  • Neutrophils have a short life period in the bloodstream (about 6-9 hours), but once they get at a region of damaged or infected tissue, they multiply rapidly. Fifty percent to seventy percent of all circulating leukocytes are mature neutrophils.
  • Chemicals called chemotaxins generated by injured cells draw neutrophils in huge numbers to the site of infection.
  • Because of their extreme mobility, neutrophils are able to diapedesically squeeze through the capillary walls of the afflicted area.
  • Neutrophils are the first type of leukocyte to arrive at sites of infection; they use chemotaxis to actively seek out invading bacteria cells, and then they engulf and digest the invaders or their waste products through the phagocytosis process.


  • Besides a nucleus with numerous lobes, neutrophils’ cytoplasm is also home to numerous tiny granules that color similarly to neutral proteins.
  • The granules’ antimicrobial ingredients include enzymes and other substances.
  • Lysosomes are specialized organelles found in neutrophils that are able to digest bacteria and other cellular waste.


  • By engulfing and destroying germs, neutrophils are an essential part of the immune response to bacterial infections.
  • As they aid in the mending of damaged tissues, they contribute to the reduction of inflammation.
  • When an infection or injury occurs, neutrophils create reactive oxygen species and release cytokines and chemokines that recruit additional immune cells to the scene.
  • Condition indicators for abnormal neutrophil numbers include bacterial infections, inflammation, and various types of leukemia.
  • Non-infectious inflammatory disorders, such as rheumatoid arthritis and systemic lupus erythematosus, also involve neutrophils.

2. Agranulocytes

  • Agranulocytes function as an immune system white blood cell (leukocyte).
  • They are made in the bone marrow and then circulate throughout the body.
  • There are two major subsets of agranulocytes, which are called lymphocytes and monocytes, respectively.
  • While monocytes are involved in innate immunity, lymphocytes are in charge of adaptive immunity.
  • Under the microscope, agranulocytes are more consistent in size and shape than their granulocyte counterparts.
  • Antigens, like viruses and bacteria, are recognized by lymphocytes, which then launch an attack.
  • Macrophages are a specialized type of monocyte that can engulf and digest foreign pathogens and cellular debris.
  • Critical to immunological monitoring and protection against infections, agranulocytes perform an important role in the immune system.
  • Conditions include viral infections, autoimmune disorders, and even some forms of leukemia can all be indicated by abnormally high or low agranulocyte levels.
  • There is evidence that agranulocytes play a role in autoimmune disorders including rheumatoid arthritis and systemic lupus erythematosus, which are not caused by infections.

a. Lymphocytes

Lymphocytes are a type of white blood cell, also known as leukocytes, that play a crucial role in the immune system. They are primarily responsible for recognizing and responding to foreign substances, such as bacteria, viruses, and other pathogens that invade the body.

  • lymphocytes have a single big nucleus in the centre of the cell. The thymus gland is responsible for the development of T cells, while the bone marrow is responsible for the development of B cells.
  • Around a third of all leukocytes are lymphocytes, the smallest type of leukocyte.
  • Lymphocytes in the blood have a greater size diversity than other leukocytes despite being relatively tiny overall.
  • The diameters of RBC-sized newly released lymphocytes are about the same as those of 9-18 m-sized mature lymphocytes.
  • Lymphocytes are primarily found in lymphatic tissues like the lymph nodes and spleen, though they do make their way into the bloodstream on occasion.
  • Precursor cells in lymphoid tissue and pluripotent stem cells in the red bone marrow both contribute to the development of lymphocytes.
  • As part of the immune system, lymphocytes play a variety of roles once they become activated.
  • Activated B cells, also called plasma cells, secrete antibodies that specifically target the antigen that originally sparked the immune response.
  • Helper T cells release substances that enlist and coordinate the efforts of other immune cells in an attack.


  • Lymphocytes are small, round cells that have a large, round nucleus and very little cytoplasm.
  • There are three main types of lymphocytes: B cells, T cells, and natural killer (NK) cells.
  • B cells and T cells have receptors on their surface that allow them to recognize and respond to specific antigens.


  • B cells are responsible for producing antibodies, which are proteins that help to identify and neutralize foreign substances such as bacteria and viruses.
  • T cells have several functions, including helping to activate other immune cells, directly killing infected cells, and regulating immune responses. NK cells are able to recognize and kill abnormal cells such as cancer cells.
  • Lymphocytes are an important component of the immune system and play a crucial role in protecting the body against infection and disease.

b. Monocytes

Certain monocytes also serve as precursor cells for other cells of the mononuclear phagocyte system in connective tissue, including macrophages, osteoclasts, and microglia.

  • The monocyte is the largest of the white blood cells, and its nucleus is kidney-shaped.
  • They’re part of the body’s innate immune system and are made in the bone marrow.
  • To become macrophages or dendritic cells, monocytes must first circulate in the circulation for several hours before moving to tissues.
  • Large in stature, monocytes have plenty of cytoplasm packed with tiny granules.
  • In order to eliminate pathogens like bacteria and viruses, the body makes use of specialised cells called monocytes.
  • Because of their speedy migration to areas of infection or injury, phagocytes play a crucial role in the earliest phases of an immune response.
  • Another function of monocytes is antigen presentation to T cells, which can aid to initiate an adaptive immune response.
  • Debris and dead cells are removed from the body with the help of monocytes.
  • The signalling molecules cytokines and chemokines they create are crucial in coordinating immune responses.
  • Tissue homoeostasis is something that monocytes help with both during development and in the maintenance of.
  • Many diseases, including autoimmune disorders, infectious diseases, and cancer, have been linked to abnormalities in monocyte levels or function.
  • Blood tests for monocytes are useful for doing things like diagnosing and keeping tabs on disorders.
  • Adaptive immune responses are initiated and controlled in large part by cells that develop from monocytes.
  • Macrophages are specialised forms of monocytes that are able to colonise tissues, where they can continue to phagocytose infections, create cytokines, and deliver antigens to T cells.
  • Dendritic cells, a kind of monocyte, are important for inducing an adaptive immune response because of their ability to capture and convey antigens to T cells.
  • Lysosomes are being formed at the Golgi apparatus, which is also home to mitochondria and some rough ER.
  • Interleukin 1 (IL-1) is produced by monocytes and stimulates B cell activation and phagocytosis, making monocytes essential for inflammation.


  • They can differentiate into a wide variety of immune cells, and can be identified by their big, kidney-shaped nucleus.
  • The diameter of circulating monocytes is 12-15 μm, but that of macrophages is typically greater.
  • Massive and typically recessed or C-shaped, the monocyte nucleus is surrounded by thick chromatin.
  • Monocytes have a basophilic cytoplasm with numerous tiny azurophilic granules that are likely lysosomes.
  • Compared to other types of white blood cells, monocytes tend to be significantly larger, and their nucleus-to-cytoplasm ratio is lower. The Golgi apparatus, which is essential for protein synthesis and modification, is also well visible in these cells.
  • Monocytes can communicate with other immune cells and respond to different signals thanks to the wide array of receptors found on their cell surface. Toll-like receptors (TLRs) are among these, and they are responsible for recognising pathogen-associated molecular patterns (PAMPs) and triggering an immune response.


  • Phagocytosis: Monocytes have the ability to engulf and digest foreign particles, such as bacteria, viruses, and other microorganisms. This process is known as phagocytosis, and it helps to protect the body against infection.
  • Antigen presentation: Monocytes can also act as antigen-presenting cells (APCs), which means they can present pieces of foreign antigens to other immune cells, such as T cells. This helps to activate an immune response against the invading pathogen.
  • Differentiation: Monocytes have the ability to differentiate into different types of immune cells, such as macrophages and dendritic cells. This allows them to carry out a variety of different functions in response to different types of infections.
  • Cytokine production: Monocytes can produce a variety of cytokines, which are signaling molecules that help to regulate the immune response. These cytokines can activate other immune cells and help to coordinate the overall response to infection.

Platelets or Thrombocytes

  • Platelets are cell fragments with an irregular shape that travel through the bloodstream until they are either activated to form a clot or eliminated by the spleen.
  • The risk of bleeding increases in those with thrombocytopenia, a disorder marked by low platelet counts. On the other hand, an abnormally large number of platelets (thrombocythemia) raises the danger of unneeded clot formation in the circulation.
  • They could cut off blood flow to vital organs including the heart and brain, leading to health problems like infarction and ischemia.
  • Platelets, like all other blood cells, develop from pluripotent stem cells in the bone marrow. The stem cells “shed” platelets into the bloodstream by differentiating into platelet precursors (called megakaryocytes). This region has a platelet circulation time of around 9 days.
  • At this time, if they come into contact with damaged blood vessel walls, they adhere to the area and get activated to form a blood clot. As a result, the opening is sealed.
  • Otherwise, when their time in the body is up, the spleen will filter them out of the bloodstream. Increased bleeding occurs when the spleen is hyperactive, as seen in conditions such as rheumatoid arthritis and leukaemia.

Structure of Platelets or Thrombocytes

  • Shape: Platelets have a discoid shape but are actually more of an irregular oblate shape. Its usual diameter is just 2–4 micrometres, making them far more diminutive than either red or white blood cells.
  • Granules: Many proteins and chemicals necessary for blood clotting are packed into granules in platelets. For platelets to coagulate in response to injury, these granules are crucial.
  • Platelet membrane: Glycoproteins on the platelet membrane facilitate adhesion to injured blood artery walls and to other platelets during the formation of the initial platelet plug.
  • Cytoplasm: Platelet cytoplasm is made up of a network of microtubules and microfilaments that serve to both keep the platelet in its current shape and allow it to transform into a clotting spheroid when needed.
  • Mitochondria: Platelets have mitochondria, which generate energy for the cell so it can do its work, but only a very tiny number of them.

Functions of Platelets or Thrombocytes

  1. Blood clotting: Platelets are essential for the formation of blood clots, which help to prevent excessive bleeding in response to injury. When a blood vessel is damaged, platelets will adhere to the site of injury and release chemicals that activate other platelets and help to form a stable clot.
  2. Wound healing: In addition to their role in blood clotting, platelets also release growth factors that stimulate the healing of damaged tissues. These growth factors can help to promote the growth of new blood vessels and new tissue formation.
  3. Immune response: Platelets also play a role in the body’s immune response. They can interact with white blood cells to help fight off infections, and they can also recognize and destroy bacteria and other pathogens.
  4. Inflammation: Platelets also release chemicals that can contribute to inflammation in the body. While inflammation is a normal part of the healing process, excessive inflammation can contribute to a range of health problems.

Functions of Blood cells

The three types of blood cells – red blood cells, white blood cells, and platelets – have different functions in the body. Here are some of their main functions:

  • Whereas white blood cells transport oxygen from the lungs to the rest of the body, red blood cells (RBCs) transfer carbon dioxide from the tissues back to the lungs to be expelled.
  • The immune system relies on white blood cells (WBCs) to fight against harmful invaders and illnesses. They aid in the detection and elimination of foreign microbes, the production of antibodies that neutralise potentially hazardous compounds, and the regulation of inflammation.
  • Platelets are crucial for blood clotting. When a blood vessel is damaged, platelets migrate to the area, clump together to create a plug, and set off a cascade of processes that eventually form a stable clot that seals the wound and stops the bleeding.
  • In addition to delivering oxygen and nutrients, RBCs transport bicarbonate ions, which aid in maintaining the body’s acid-base balance.
  • White blood cells (WBCs) aid in the elimination of dead cells and other detritus while also aiding in the restoration of healthy tissue.
  • The growth factors released by platelets promote tissue and blood vessel repair and regeneration.
  • Regular blood cells (RBCs) have a role in the control of blood pressure and blood flow because they transport nitric oxide (a vasodilator).
  • White blood cells (WBCs) aid in controlling body temperature and preserving fluid and electrolyte balance.
  • When blood artery walls are healthy, excessive bleeding is less likely to occur, thanks to platelets.
  • Each of the three types of blood cells has a role in distributing vital nutrients and hormones to the body’s many organs and tissues.


What are blood cells?

Blood cells are the cells that make up blood. They include red blood cells, white blood cells, and platelets.

What is the function of white blood cells?

White blood cells, or leukocytes, are part of the immune system and help the body fight off infections and diseases.

What is the function of red blood cells?

Red blood cells, or erythrocytes, carry oxygen from the lungs to the rest of the body and carry carbon dioxide from the body to the lungs to be exhaled.

Where are blood cells made?

Blood cells are made in the bone marrow, which is the spongy tissue inside bones.

How long do blood cells live?

The lifespan of blood cells varies by type. Red blood cells typically live for about 120 days, white blood cells can live for a few hours to a few days, and platelets live for about 7-10 days.

What happens when someone has a low red blood cell count?

A low red blood cell count, also known as anemia, can cause fatigue, weakness, shortness of breath, and other symptoms.

What happens when someone has a high white blood cell count?

A high white blood cell count, also known as leukocytosis, can be a sign of infection, inflammation, or other medical conditions.

Can blood cells be donated?

Yes, blood cells can be donated through blood donation. Red blood cells, platelets, and plasma can all be donated to help patients in need.

Can blood cells be replaced?

Yes, blood cells can be replaced through a bone marrow transplant. This is a medical procedure in which healthy stem cells are transplanted into a patient to replace damaged or diseased bone marrow.


  • Blumenreich MS. The White Blood Cell and Differential Count. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths; 1990. Chapter 153. Available from: https://www.ncbi.nlm.nih.gov/books/NBK261/
  • Barbalato L, Pillarisetty LS. Histology, Red Blood Cell. [Updated 2022 Nov 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539702/
  • Dean L. Blood Groups and Red Cell Antigens [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2005. Chapter 1, Blood and the cells it contains. Available from: https://www.ncbi.nlm.nih.gov/books/NBK2263/
  • http://qu.edu.iq/el/mod/resource/view.php?id=60981
  • https://basicmedicalkey.com/blood-3/
  • https://mybiblioteka.su/10-80714.html
  • https://www.statpearls.com/articlelibrary/viewarticle/28286/


Submit Your Question
Please submit your question in appropriate category.

Leave a Comment