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Immunity – Definition, Types

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What is Immunity?

Immunity is the body’s capacity to fight itself against germs that cause disease. Every day, our bodies are exposed to a variety of pathogens, but only a few of them result in disease. Our body has the ability to release antibodies against certain viruses, so protecting it from sickness. This protective process is known as immunity.

  • The term immunity derives from the Latin word “immunis” (exempt), which originally referred to the legal protection granted to Roman senators throughout their terms in office.
  • This phrase was eventually adopted to refer to the protection obtained naturally against illnesses such as measles and smallpox. It demonstrated that a person can gain permanent resistance to a particular disease after contracting it once.
  • The immune system is comprised of the cells and chemicals responsible for immunity, and their aggregate and coordinated response to foreign substances is known as the immunological response.
  • The concept of immunity dates back to antiquity. The Chinese custom of making infants resistant to smallpox by having them inhale powders formed from the skin lesions of healing patients is one example.
  • During the fifth century B.C., Thucydides in Athens documented the earliest reference of immunity in Europe.
  • At 430 B.C., when discussing the epidemic in Athens, he wrote that only those who had recovered from the disease could care for the ill, as they would not contract it again.
  • Once the concept of immunity’s existence was established, manipulation of immunity under controlled conditions quickly followed.
  • First, Edward Jenner conducted a successful experiment in which he injected material from a cowpox pustule into the arm of an 8-year-old kid and established that the boy did not develop smallpox despite future exposure.
  • Based on his observations, milkmaids who had cowpox never caught the more severe smallpox.
  • Jenner’s method of vaccinating against smallpox with cowpox swiftly spread throughout Europe.
  • However, for numerous reasons, including a lack of understanding of clear disease targets and their causes, it took nearly a century for this method to be used to prevent smallpox.
  • In 1901, von Behring was awarded the Nobel Prize in Medicine for his 1890 experiments with Shibasaburo Kitasato, which provided the first insights into the mechanism of immunity.
  • Von Behring and Kitasato discovered that serum (the liquid, noncellular component of coagulated blood) from animals previously inoculated against diphtheria might transfer immunity to animals not previously immunised.
  • Since then, immunology has advanced significantly as a subject of research. Immunological research has been and continues to be one of the most popular topics of study, as evidenced by the fact that about 17 Nobel Prizes have been awarded to researchers in the field.

Immune System

  • The immune system is the body’s most effective defence mechanism. It protects us from harmful bacteria and keeps us healthy.
  • Immunology is a discipline of biology that focuses on the intricate activities of the immune system. Immunity is the capacity to combat antigens or pathogens and the state of being healthy.
  • The immune system consists of cells, tissues, and organs that protect the body as a whole.
  • This system defends the human body in numerous ways against invading germs.
  • Some of these abilities are innate, while others are acquired. Consequently, they serve a purpose in allergy, autoimmunity, and organ transplantation.
  • White blood cells (or leukocytes) are the most essential cells in the immune system because they kill disease-causing germs or chemicals.
  • In addition to leukocytes, the immune system also includes lymphoid organs, tissues, and proteinaceous molecules called antibodies.

Lymphoid Organs

  • Lymphoid organs refer to the immune system organs that defend the body against invading microorganisms that cause illnesses or the spread of cancer.
  • It consists of bone marrow, blood vessels, lymph nodes, lymphatic vessels, the thymus, the spleen, and other lymphoid tissue clusters.
  • Lymphoid organs are where lymphocytes originate, mature, and proliferate. They are classified as primary, secondary, or tertiary according to their degree of development and maturity.
  • These organs are composed of fluid connective tissues containing various leukocytes or white blood cells. White blood cells or leukocytes contain the highest proportion of lymphocytes.

Primary lymphoid organs

  • The primary lymphoid organs create lymphocytes and enable for their development. Additionally, it produces lymphocytes from immature progenitor cells.
  • Consequently, it is known as the core lymphoid organs. Among the principal lymphoid organs are the thymus and bone marrow.

Secondary lymphoid organs

  • The secondary lymphoid organs are referred to as peripheral lymphoid organs because they promote the places where lymphocytes interact with antigen to become effector cells. They launch an adaptive immune response.
  • These are the secondary lymphoid organs The spleen, tonsils, lymph nodes, and appendix, among others, are examples of secondary lymphoid organs.

Tertiary lymphoid organs

  • Typically, tertiary lymphoid organs contain a small number of lymphocytes. It plays a crucial function in the inflammatory process.

How does Immunity Work?

  • To comprehend the immunity phenomena, it was necessary to comprehend the immunity mechanism; consequently, the experimental work of Emil von Behring and Shibasaburo Kitasato in 1890 provided the first insight into the immunity mechanism.
  • They demonstrated that serum included antibody-like substances.
  • They protected against infections, so setting the groundwork for the discovery of humoral immunity.
  • In appreciation of his work, Emil von Behring earned the Nobel Prize in Medicine in 1901.
  • Elie Metchnikoff proved that cells contribute to an animal’s immunological state in 1884, prior to von Bekring’s demonstration of the serum components. Metchnikoff noticed that certain white blood cells, which he dubbed phagocytes, could consume bacteria and other foreign material (phagocytose).
  • He observed that phagocytic cells were very active in immunised animals and concluded that cells rather than serum components were the primary mediators of immunity. Metchnikoff recognised active phagocytic cells in the blood as monocytes and neutrophils.

Types of Immunity

The primary purpose of the immune system is to prevent or restrict infections caused by harmful microorganisms such as bacteria, viruses, parasites, and fungi. A host’s immune responses commence with the identification of bacteria and foreign chemicals. The body’s defence systems are (a) innate (natural) immunity and (b) acquired (adaptive) immunity.

1. Innate Immunity

  • This sort of immunity is present at birth in an organism.
  • This is immediately engaged when a pathogen is detected. Certain barriers and defence systems that keep foreign particles out of the body are a part of innate immunity.
  • Innate immunity is the body’s defence mechanism.
  • This immunity benefits us by supplying the natural resistance components, such as salivary enzymes, natural killer cells, undamaged skin, and neutrophils, etc., that form an early reaction against illnesses at birth before exposure to pathogens and antigens.
  • It is a form of long-term immunity in which our body develops its own antibodies. Few natural barriers prevent viruses from entering the body.

What is Innate immune responses?

Innate immunity is regarded as the host’s initial line of protection against foreign agents. There are five categories of innate immune response components:

  • The removal of foreign agents from body tissues by macrophages, which are non-specific white blood cells.
  • By generating anti-inflammatory mediators known as cytokines and chemokines, immune cells are attracted to the site of infection.
  • The complement cascade is a chain of immunological processes that functions to eliminate germs, infected host cells, and debris.
  • Additionally, innate immunity serves as a physical barrier against the entrance of foreign agents (e.g. skin is a component of the innate immune system).
  • By delivering foreign, processed antigens to adaptive immunity, the innate immune response activates the adaptive immune system.
Innate Immunity 
Innate Immunity 

Cells Involved In Innate Immunity

  • Phagocytes: These cells circulate throughout the body and search for foreign substances. They envelop and destroy the infection to protect the body against it.
  • Macrophages: These cells are capable of traversing the walls of the circulatory system. They secrete cytokines to recruit additional cells at the location of an infection.
  • Mast cells: Mast cells are essential for wound healing and defending against infections.
  • Neutrophils: Neutrophils have naturally poisonous granules that kill any infection with which they come into contact.
  • Eosinophils: Eosinophils contain highly poisonous proteins that eliminate any germs or parasites that come into touch with them.
  • Basophils: These cells target multicellular parasites. Similar to mast cells, these cells produce histamine.
  • Natural Killer Cells: These destroy infected host cells to prevent the spread of diseases.
  • Dendritic Cells: These are found in the tissues that serve as first infection entry locations. By antigen presentation, these cells alert the rest of the immune system to the presence of an infection.

Types of Barriers

The four types of barriers are:


a. Physical barrier

  • This includes the skin, body hair, cilia, eyelashes, respiratory system, and digestive system.
  • These constitute the initial line of defence.
  • The skin provides more than just a fair or dark complexion. Our skin serves as a physical barrier against virus infiltration.
  • The mucus coating in the nose and ear acts as a protective barrier that captures pathogens before they can enter the body.

b. Physiological barriers

  • We know that hydrochloric acid is used by the stomach to break down food molecules.
  • Due to such a highly acidic environment, the majority of pathogens that enter the body with meals are eliminated prior to further processing.
  • Saliva in our mouth and tears in our eyes both have the antibacterial property that prevents the growth of germs despite constant exposure.

c. Cellular barriers

  • Despite physical and physiological barriers, some infections are able to infiltrate the human body.
  • Leukocytes (WBC), neutrophils, lymphocytes, basophils, eosinophils, and monocytes comprise this barrier. Every one of these cells is present in the blood and tissues.

d. Cytokine barriers

  • Our body’s cells are more intelligent than we give them credit for. For instance, whenever a cell in our body is invaded by a virus, it automatically secretes interferons, which form a coating around the diseased cell and inhibit further infection of the surrounding cells.

2. Acquired Immunity

  • Adaptive immunity, also known as acquired immunity, is the immunity that our body gains or acquires over time. This, unlike innate immunity, is not present at birth.
  • Acquired immunity is the capacity of the immune system to adapt to illness and create pathogen-specific immunity. This is also called adaptive immunity.
  • An individual gets immunity after birth; hence, it is known as acquired immunity.
  • It is particular and mediated by lymphocytes or antibodies that render the antigen harmless.
  • The primary goal of acquired immunity is to alleviate the symptoms of an infectious disease and prevent subsequent attacks.
  • It comprises primarily of a sophisticated lymphatic defence system that recognises and does not react to the body’s own cells.
  • The immune system of our body recognises germs it has already encountered. It is generally caused by direct contact with the infection or its antigen.
  • Our body produces antibodies to engulf and destroy the pathogen’s antigen.
  • When something is encountered for the first time, it is referred to as a primary response.
  • Once the body becomes accustomed to certain pathogens, antibodies are prepared to battle them again; this is known as naturally acquired immunity.

Features of Acquired Immunity

  • Specificity: Our body is able to distinguish between different sorts of infections, whether they are hazardous or not, and design methods to eliminate them.
  • Diversity: Our immune system can detect a wide range of infections, from protozoa to viruses.
  • Differentiate between self and non-self: Our body is able to distinguish between its own cells and foreign ones. It begins rejecting any foreign cells in the body instantly.
  • Memory: Once the body detects a pathogen, it activates the immune system to eliminate it. It also recalls which antibodies were produced in response to a particular infection so that, the next time the pathogen enters the body, a similar process is followed to destroy it.

What is Adaptive immune Responses?

  • This is the body’s most specialised immune response.
  • With the support of two primary immune cells, B-cells and T-cells, it seeks to eliminate previously encountered foreign invaders.
  • B cells are responsible for the release of foreign antigen-specific antibodies.
  • T cells are responsible for both activating B cells (T helper cells) and destroying pathogens/host cells that have been infected by pathogens (T-killer cells).
  • This branch of the immune system is adaptive due to the multiple processes it employs to mature immune cells, so rendering them highly specific to antigens encountered on infections and presented by the innate immune system.
Acquired Immunity
Acquired Immunity

Cells Involved in Acquired Immunity

There are two types of cells involved in acquired immunity: B-cells and T-cells.


  • They are generated in the bone marrow.
  • These cells become active upon contact with external substances. These alien particles serve as identifiers for foreignness.
  • B-cells rapidly develop into plasma cells that produce antibodies specific to the so-called antigen or foreign particle.
  • These antibodies connect to the antigen/foreign agent’s surface.
  • These antibodies can identify and eliminate any antigen in the body.
  • Immunity reliant on B-cells is known as humoral immunity.


  • They form in the thymus after developing in the bone marrow.
  • Helper cells, cytotoxic cells, and regulatory cells are the products of T-cell differentiation. The release of these cells into the bloodstream.
  • When activated by an antigen, these cells release cytokines that function as messengers.
  • These cytokines initiate the development of B-cells into plasma cells, which thereafter release antigen-specific antibodies.
  • The cancer cells are killed by cytotoxic T-cells.
  • Regulatory T-cells control immunological responses.

Types of Acquired Immune Response

Humoral Immune Response

  • Antibodies generated by B-lymphocytes are found in blood cells and are carried throughout the body. The humoral immune response is composed of an antibody produced by lymphocytes; hence, its name.
  • It depends on the action of circulating antibodies in the body. Humorous immunity is activated when an antibody on a B-cell connects with an antigen. The B cell internalises the antigen and presents it to the T cell helper. This stimulates B-cells.
  • Activated B cells proliferate and generate plasma cells.
  • These plasma cells secrete antibodies into the circulatory system. The memory B cells maintain information about the infection in order to prevent future diseases caused by that pathogen.

Cell-mediated Immune Response

  • T helper cells start the process of cell-mediated immunity.
  • By producing toxins and consequently causing apoptosis or programmed cell death, cytotoxic T cells eradicate infected cells from the body.
  • T helper cells contribute to the activation of other immune cells. In the case of transplant patients, cell-mediated immunity becomes obvious.
  • When one of our sense organs ceases to function, another organ can be transplanted to replace it. However, this is not the case with the immunological response. It suggests that T-lymphocytes are capable of distinguishing between native and foreign tissues and organs.
  • Because our body may reject the transplanted organ, even if we find a donor with the same blood group, we cannot transplant and implant organs into our bodies.
  • T-cells instantly identify the tissue or organ as alien and prevent it from becoming a part of the body.
  • This is the reason why transplant recipients must take immunosuppressants for the rest of their lives.
  • T-lymphocytes are responsible for controlling this response.

Types of Acquired Immunity

Active Immunity

  • Active immunity is the immediate response of the body to a foreign antigen.
  • In the acquired or adaptive immune system, the body recalls the infections it has previously faced. This is directly attributable to an active immune system.
  • Active immunity occurs when the disease or its antigen is encountered.
  • Antigens stand for antibody generator.
  • With the aid of antigens secreted by the disease, our body is able to combat the pathogen.
  • Consequently, based on the antigen of the virus, our body begins generating antibodies to combat it.
  • When this occurs initially, it is referred to as a primary response.
  • Once the body has encountered a virus for the first time, it retains a few of the antibodies that fought it in case it assaults again. The term for this is natural active immunity.

Passive Immunity

  • Passive immunity is the immunological response caused by antibodies acquired from the outside of the body. The body’s initial response to a disease it encounters for the first time is quite poor, hence the initial encounter is always somewhat taxing on the body.
  • What if everyone could be immunised without ever having to become sick? Over the past two decades, biotechnology has advanced significantly, and we are now able to manufacture antibodies against diseases. Even if the body has not yet mounted a major immune response, these premade antibodies offer protection.
  • Active immunity may provide lifelong protection against a disease, but passive immunity is more temporary.
  • Passive immunity develops instantly, allowing our body to quickly begin attacking the invader.
  • Two types of passive immunity exist:
    • Natural Passive Immunity
    • Artificial Passive Immunity
Innate and Adaptive Immunity
Innate and Adaptive Immunity


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