Antigens are any substances that stimulate the immune system. Foreign antigens originate from the outside of the body and may be pathogens (such as disease-causing viruses or bacteria), allergens (such as pollen), or toxins (such as venom or chemicals). Autoantigens originate within the body and typically do not elicit an immune response, except in people with autoimmune disorders.

Produced by white blood cells, an antibody is a Y-shaped protein that marks antigens for destruction by immune cells. A complex of antigen and antibody is formed when an antibody binds to an antigen. The formation of an antigen-antibody complex provokes an immune response against the foreign substance.

What is an Antigen?

• An antigen is a substance that elicits an immune response within the body. These substances can be classified into two main categories: foreign antigens and autoantigens.
• Foreign antigens encompass various substances that originate from the external environment. They can be produced by disease-causing viruses, bacteria, or other microorganisms commonly known as pathogens. Additionally, foreign antigens may include venom from insects or snakes, pollen, chemicals, or specific proteins found in certain foods. When these substances enter the body, they are recognized as “not-self” by the immune system, which triggers a defensive response to combat and eliminate them.
• Autoantigens, on the other hand, are generated within the body itself. Normally, the immune system distinguishes these self-antigens as part of the body’s own components and does not initiate an immune response against them. However, in individuals with autoimmune disorders, a malfunction occurs, and the body fails to differentiate between self-antigens and foreign antigens. Consequently, the immune system mistakenly launches an attack against its own tissues and cells, leading to detrimental consequences.
• In autoimmune conditions, the body produces autoantibodies, which are antibodies that target and attack the self-antigens. This misguided immune response causes damage to various cells and tissues throughout the body. The extent of the damage varies depending on the specific autoimmune disorder and the affected organs or systems.
• Understanding antigens is crucial in comprehending the mechanisms behind immune responses and autoimmune diseases. The identification and characterization of specific antigens play a pivotal role in diagnosing autoimmune disorders and developing targeted treatment approaches. Researchers continue to explore the intricate interplay between antigens and the immune system, aiming to unravel the complexities of autoimmune conditions and discover novel therapeutic interventions.

Characteristics of Antigen

Antigens possess several key characteristics that define their role in the immune system. These characteristics include:

1. Foreignness: Antigens are typically foreign substances to the body. They can originate from pathogens like bacteria, viruses, fungi, and parasites, as well as non-living entities such as toxins and chemicals. The immune system recognizes these foreign antigens as “non-self” and mounts an immune response against them.
2. Immunogenicity: Antigens are capable of triggering an immune response. They possess specific molecular structures, known as epitopes or antigenic determinants, which are recognized by the immune system. These epitopes can be unique to each antigen or shared among different antigens.
3. Molecular Complexity: Antigens can have diverse molecular compositions and complex structures. They may consist of proteins, carbohydrates, lipids, or nucleic acids. The complexity of antigens influences the specificity and strength of the immune response they elicit.
4. Specificity: Antigens are recognized by the immune system in a highly specific manner. The immune system produces antibodies or immune cells (such as T cells) that specifically bind to the antigenic epitopes on the surface of the antigen. This binding occurs through molecular interactions between the antigen and the antigen receptor of immune cells or antibodies.
5. Immunological Memory: Antigens can induce the formation of memory cells within the immune system. After an initial exposure to an antigen, memory cells are generated, enabling a more rapid and robust immune response upon subsequent encounters with the same antigen. This immunological memory contributes to long-term immunity against specific antigens.
6. Self-Nonself Discrimination: The immune system can distinguish between self-antigens (antigens derived from the body’s own tissues) and foreign antigens. Normally, the immune system is tolerant to self-antigens, avoiding immune responses against them. However, in certain conditions like autoimmune diseases, this self-nonself discrimination may be disrupted, leading to immune reactions against self-antigens.
7. Variability: Antigens can exhibit variability, either within a pathogen or among different strains or species. This variability allows pathogens to evade immune recognition or contribute to the diversity of immune responses generated against them.

Examples of Antigen

Antigens can encompass a wide range of substances originating from pathogens, allergens, toxins, and even self-components. Here are some examples of antigens:

1. Pathogen-derived Antigens: These antigens are derived from various disease-causing microorganisms, including:a. Viral Antigens: Proteins found on the surface of viruses, such as the spike protein of SARS-CoV-2 (the virus responsible for COVID-19).b. Bacterial Antigens: Components of bacterial cells, such as lipopolysaccharides (LPS) in gram-negative bacteria or proteins like flagellin.c. Fungal Antigens: Cell wall components of fungi, such as β-glucans or mannoproteins.d. Parasitic Antigens: Proteins or glycoproteins expressed by parasites, like the malarial antigen found on Plasmodium falciparum.
2. Allergens: These antigens trigger allergic responses in susceptible individuals. Common examples include:a. Pollen Allergens: Antigens found in pollen grains, which can cause seasonal allergies.b. Dust Mite Allergens: Proteins from dust mites that can elicit allergic reactions.c. Pet Dander Allergens: Antigens present in the skin cells, saliva, or urine of pets, which can cause allergies in sensitive individuals.
3. Transplant Antigens: These antigens are involved in immune recognition of transplanted organs or tissues. They include: a. Human Leukocyte Antigens (HLA): A group of cell surface proteins involved in tissue compatibility and immune response modulation.
4. Autoantigens: Self-antigens that can trigger autoimmune responses when the immune system mistakenly recognizes them as foreign. Examples include:a. Insulin Autoantigen: In type 1 diabetes, the immune system targets insulin-producing cells in the pancreas.b. Thyroid Autoantigens: In autoimmune thyroid diseases like Hashimoto’s thyroiditis or Graves’ disease, the immune system targets thyroid-specific proteins.
5. Blood Group Antigens: Antigens present on the surface of red blood cells that determine blood type. Examples include the ABO and Rh blood group antigens.
6. Tumor-associated Antigens: These antigens are expressed on cancer cells and can be recognized by the immune system. They can serve as targets for immunotherapies or cancer vaccines.

It’s important to note that these examples represent only a fraction of the antigens encountered in the immune system. Antigens can be diverse in nature and can elicit specific immune responses, leading to immune protection or, in certain cases, immune-mediated diseases.

What is an Antibody?

• An antibody, also known as an immunoglobulin, is a specialized protein produced by the immune system in response to the presence of foreign substances, known as antigens.
• When the body is exposed to an antigen, such as a pathogen or a foreign molecule, B cells, a type of white blood cell, play a vital role in the production of antibodies. Each B cell is equipped with unique receptors on its surface that recognize and bind to specific antigens. Once a B cell encounters an antigen that matches its receptors, it becomes activated and begins to proliferate.
• As the B cell multiplies, it undergoes a process called differentiation, where it develops into plasma cells. Plasma cells are antibody-producing factories that generate a large quantity of specialized proteins tailored to recognize and bind to the specific antigen encountered by the B cell. These proteins are the antibodies.
• Antibodies are highly specific in their function. Each type of antibody is designed to recognize and bind to a particular antigen, forming an antigen-antibody complex. The structure of an antibody consists of four polypeptide chains, which form a Y-shape. Two of these chains, known as heavy chains, are longer and provide structural support, while the other two, called light chains, are shorter. The constant region of the antibody remains the same among individuals, whereas the variable region, located at the tips of the Y-shaped arms, differs and determines the antibody’s specificity.
• The antigen-binding site, located within the variable region, is where the antibody binds to its target antigen with high affinity and specificity. This binding interaction between the antibody and antigen is crucial for initiating a series of immune responses against the foreign substance. Once bound, antibodies can neutralize the antigen by preventing its interaction with host cells, mark it for destruction by other immune cells, or initiate a cascade of immune reactions that ultimately eliminate the antigen from the body.
• The production of antibodies is a fundamental defense mechanism of the immune system. By recognizing and binding to antigens, antibodies assist in protecting the body from harmful substances, such as pathogens or toxins, thereby preventing or reducing the likelihood of illness. The ability of antibodies to specifically recognize and target antigens forms the basis for various diagnostic tests, therapeutic interventions, and vaccine development.

Characterisitcs of Antibody

Antibodies, also known as immunoglobulins, possess several characteristic features that contribute to their role in the immune system. Here are some key characteristics of antibodies:

1. Protein Nature: Antibodies are proteins produced by B cells, a type of white blood cell (lymphocyte). They belong to the immunoglobulin superfamily and are composed of polypeptide chains.
2. Y-shaped Structure: Antibodies have a characteristic Y-shaped structure, consisting of four polypeptide chains held together by disulfide bonds. Each antibody molecule has two identical heavy chains and two identical light chains, forming the arms of the Y.
3. Variable and Constant Regions: Antibodies have distinct regions within their structure. The variable regions, located at the tips of the Y arms, contain unique amino acid sequences that allow antibodies to recognize and bind to specific antigens. The constant regions, found in the stem of the Y, determine the antibody’s class and function.
4. Specificity: Antibodies exhibit high specificity. Each antibody is designed to bind to a specific epitope or antigenic determinant on an antigen. The variable regions of the antibody provide the antigen-binding sites, allowing antibodies to recognize and bind to their target antigens with high affinity.
5. Diversity: The immune system generates a vast array of antibodies capable of recognizing and binding to a wide range of antigens. This diversity is achieved through genetic recombination and somatic hypermutation during B cell development, resulting in the production of millions of unique antibody molecules.
6. Isotype and Subclasses: Antibodies can be classified into different isotypes, including IgM, IgG, IgA, IgD, and IgE, based on their constant region structure. These isotypes have distinct functions and distribution within the body. For example, IgM is the first antibody produced during an initial immune response, while IgG provides long-term immunity and is the most abundant antibody in circulation.
7. Effector Functions: Antibodies can initiate various effector functions to combat pathogens or foreign substances. These include neutralization, opsonization, complement activation, antibody-dependent cellular cytotoxicity (ADCC), and antibody-mediated cellular internalization.
8. Immunological Memory: After an initial encounter with an antigen, the immune system generates memory B cells that can produce a rapid and robust antibody response upon re-exposure to the same antigen. This immunological memory contributes to long-term immunity.
9. Half-life and Clearance: Antibodies have varying half-lives, ranging from a few days to several weeks, depending on the antibody class. They can be cleared from the body through various mechanisms, including degradation, filtration by the kidneys, and binding to specific receptors on immune cells.
10. Production Regulation: Antibody production is tightly regulated by complex signaling pathways involving interactions between B cells, T cells, and antigen-presenting cells. This regulation ensures an appropriate immune response and prevents excessive antibody production.

Examples of Antibody

There are several types of antibodies (immunoglobulins) that serve different functions in the immune system. Here are some examples of antibodies:

1. IgM (Immunoglobulin M): IgM is the first antibody produced during an initial immune response. It is a pentameric antibody, meaning it consists of five antibody subunits joined together. IgM is effective at activating the complement system and is involved in neutralizing pathogens.
2. IgG (Immunoglobulin G): IgG is the most abundant antibody in the bloodstream and is responsible for long-term immunity. It can cross the placenta, providing passive immunity to newborns. IgG can neutralize pathogens, enhance phagocytosis, and activate the complement system.
3. IgA (Immunoglobulin A): IgA is found in body secretions, such as saliva, tears, breast milk, and mucosal surfaces. It plays a crucial role in preventing pathogens from entering the body through mucous membranes. IgA can also neutralize toxins and pathogens.
4. IgE (Immunoglobulin E): IgE is involved in allergic reactions and plays a role in defense against parasitic infections. It binds to mast cells and basophils, triggering the release of inflammatory mediators when encountering an allergen.
5. IgD (Immunoglobulin D): IgD is primarily found on the surface of B cells and serves as a receptor for antigen recognition. Its precise function is still not fully understood, but it may play a role in the activation of B cells.

Monoclonal antibodies are another class of antibodies that are engineered to be highly specific to a particular antigen. They are widely used in diagnostic tests, therapeutic treatments, and research. Some examples of monoclonal antibodies include:

• Rituximab: Used for treating certain types of lymphomas and autoimmune diseases by targeting B cells expressing a specific antigen called CD20.
• Trastuzumab: Used for treating HER2-positive breast cancer by targeting the HER2 receptor on cancer cells.
• Pembrolizumab: An immune checkpoint inhibitor that targets the PD-1 receptor on immune cells, enhancing the immune response against cancer cells.

These examples highlight the diversity and specificity of antibodies, each designed to recognize and bind to specific antigens and serve various functions in the immune response.

Difference between Antigen and Antibody

S.N.	Characteristics	Antigen	Antibody
1	Molecule Type	Usually proteins; may also be polysaccharides, lipids, or nucleic acids	Proteins
2	Definition	Substances that provoke an immune response	Glycoproteins secreted by immune cells (plasma cells) in response to a foreign substance (antigen)
3	Effect	Cause disease or allergic reactions	Protect the system by lysing antigenic material
4	Origin	Within the body or externally	Within the body
5	Parts	Highly variable; composed of different epitopes	Composed of two light chains and two heavy chains
6	Prevalence	Found in all types of cells, mostly in viruses, bacteria, and fungi	Present only in some types of cells
7	Synonyms	Immunogens	Immunoglobulins
8	Specific binding site	Epitope	Paratope
9	Complexity	Medium; exists due to random mutations in genes	Very high; complex chemical that binds to a specific antigen
10	Source	Foreign substances (viruses, bacteria, fungal toxins, etc.)	Naturally produced by the body
11	Kinds	Three basic kinds: Exogenous, Endogenous, Autoantigens	Five basic kinds: IgG, IgM, IgA, IgE, IgD
12	Examples	Exogenous antigens: bacteria, viruses, fungi, etc. Endogenous antigens: Blood group antigens, HLA, etc. Autoantigens: Nucleoproteins, nucleic acids, etc.	Breast milk, tears, saliva, sweat, and mucus
13	Production	Produced by pathogens, allergens, toxins, and foreign substances	Produced by B cells in response to the presence of antigens
14	Detection	Detected by the immune system through B and T cell recognition	Detected through serological tests or laboratory techniques
15	Half-life	Variable; can range from hours to years depending on the antigen	Variable; can range from days to weeks depending on the antibody class
16	Function	Induce an immune response and mark antigens for destruction	Defend against pathogens, neutralize toxins, activate complement system, promote phagocytosis, etc.
17	Location	Found on the surface of pathogens or circulating in body fluids	Circulate in the blood and other bodily fluids, and can also be found on the surface of B cells
18	Role in Allergy	Can trigger allergic reactions by binding to allergens	Involved in the allergic response by triggering the release of inflammatory mediators
19	Role in Vaccines	Used in vaccines to stimulate the immune system against specific pathogens	Used as a key component in passive immunization and therapeutic treatments
20	Molecular Size	Varied molecular sizes, ranging from small peptides to large protein complexes	Relatively large proteins with a molecular weight of approximately 150 kDa

What is an Antigen-Antibody Complex?

• An antigen-antibody complex, also referred to as an immune complex, is formed when an antibody binds to a specific antigen. This interaction between the antigen and the antibody creates a complex that plays a crucial role in initiating and modulating immune responses.
• The process of antigen-antibody complex formation begins when an antibody, produced by specialized immune cells called B cells, recognizes and binds to its corresponding antigen. The binding occurs due to the complementary shape and structure of the antigen-binding site on the antibody and the antigen itself. This binding is highly specific, meaning that each antibody can only bind to a particular antigen.
• Once the antigen and antibody have formed a complex, several important immune responses can be initiated. These responses serve to neutralize or eliminate the antigen and its associated threat to the body. The immune complex can trigger various mechanisms, depending on the nature of the antigen and the type of immune response required.
• One mechanism is neutralization, where the antigen-antibody complex prevents the antigen from interacting with host cells or tissues. By binding to the antigen, the antibody can block its harmful effects and render it harmless. This neutralization can occur for various types of antigens, including toxins, viruses, and bacteria.
• Another mechanism is opsonization, where the immune complex marks the antigen for recognition and destruction by other components of the immune system. The bound antibody acts as a flag, signaling to immune cells such as macrophages and neutrophils that the antigen is foreign and needs to be eliminated. These immune cells possess receptors that specifically recognize the constant region of antibodies, allowing them to engulf and eliminate the antigen-antibody complex.
• Furthermore, the formation of antigen-antibody complexes can activate the complement system, a cascade of proteins that helps to enhance immune responses. The binding of antibodies to antigens triggers a series of enzymatic reactions within the complement system, leading to the formation of membrane attack complexes that can directly lyse and destroy cells, particularly bacteria.
• While antigen-antibody complexes are crucial for immune defense, in certain situations, they can also contribute to autoimmune diseases. In autoimmune disorders, the immune system mistakenly targets self-antigens, leading to the formation of immune complexes. These complexes can deposit in tissues, triggering inflammation and tissue damage.
• In summary, the formation of antigen-antibody complexes is a fundamental process in the immune response. By binding to specific antigens, antibodies play a pivotal role in neutralizing and eliminating pathogens, marking them for destruction, and activating immune mechanisms. Understanding the dynamics of antigen-antibody complexes is vital for the development of diagnostic tests, therapeutic interventions, and the advancement of our knowledge in immunology.

Antibodies and Immunity

• Antibodies play a crucial role in the immune response and the development of immunity against pathogens. When the body is exposed to a pathogen for the first time, specialized immune cells recognize the antigens on the pathogen’s surface and produce specific antibodies in response.
• These antibodies are designed to bind to the antigens of the pathogen, forming antigen-antibody complexes. This binding process helps in several ways. Firstly, it can neutralize the pathogen, rendering it unable to infect host cells or cause harm. Secondly, the antibody coating on the pathogen can facilitate its recognition and destruction by other immune cells, such as macrophages and natural killer cells.
• As the immune system eliminates the pathogen, a few B cells that produced the specific antibodies transform into memory cells. These memory cells persist in the body even after the infection has been cleared, “remembering” the specific antigens of the pathogen. This memory response is crucial for the development of immunity.
• If the person encounters the same pathogen again in the future, the memory cells quickly recognize the antigens and initiate a rapid response. The memory cells are primed to produce a large quantity of specific antibodies, enabling a swift and efficient immune reaction against the pathogen. This accelerated response allows the immune system to eliminate the pathogen before it can cause significant illness. As a result, the person is considered immune to that particular pathogen.
• The presence of memory cells and the ability to mount a robust immune response upon re-exposure to a pathogen is the foundation of acquired immunity. This type of immunity provides long-term protection against specific pathogens and is the principle behind vaccination. Vaccines stimulate the immune system to produce memory cells without causing the full-blown disease, thereby preparing the body to mount a swift and effective immune response if it encounters the pathogen in the future.
• Antibodies and the immune memory they create are essential components of the immune system’s defense against pathogens. They contribute to the body’s ability to fight off infections more efficiently, reducing the severity and duration of illness. The development of vaccines and understanding the immune memory response have revolutionized medicine, enabling the prevention and control of many infectious diseases.

What is an Antibody Test?

• An antibody test, also known as a serology test, is a diagnostic tool used to determine if an individual has been previously exposed to a specific pathogen. This type of test detects the presence of antibodies in a blood sample, indicating whether the person has mounted an immune response against the particular pathogen.
• When the immune system encounters a pathogen for the first time, it recognizes the antigens present on the surface of the pathogen as foreign. In response, specialized immune cells called B cells produce antibodies that are specifically designed to target and neutralize the pathogen. These antibodies circulate in the bloodstream and other bodily fluids, providing a defense mechanism against the pathogen.
• During an antibody test, a sample of blood is collected from the individual. This blood sample is then examined for the presence of specific antibodies that correspond to the antigens of the pathogen being tested for. The test typically involves using laboratory techniques to detect and identify these antibodies.
• If the person has previously been exposed to the pathogen, the immune system will have generated antibodies in response. These antibodies will be detectable in the blood sample, indicating prior exposure. The presence of antibodies suggests that the person has mounted an immune response against the pathogen, whether it resulted in illness or not.
• It’s important to note that antibody tests are not used to diagnose acute or current infections. Instead, they provide information about past exposure to a particular pathogen. Antibodies can persist in the body for a variable period, depending on the pathogen and the individual’s immune response. Therefore, antibody tests are particularly useful for assessing previous infections or determining the prevalence of a pathogen within a population.
• Antibody tests have been widely employed for various infectious diseases, including viral infections like COVID-19, HIV, hepatitis, and bacterial infections like Lyme disease. They are also utilized for assessing immune responses to vaccinations, monitoring the effectiveness of treatment, and conducting epidemiological studies.
• In summary, an antibody test is a diagnostic tool that examines a blood sample for the presence of specific antibodies against a particular pathogen. It provides information about previous exposure to the pathogen and can be used for various purposes, such as assessing immunity, monitoring disease prevalence, and evaluating vaccine responses.

References

• https://microbiologyinfo.com/differences-between-antigen-and-antibody/
• https://pediaa.com/difference-between-antigen-and-antibody/
• https://microbenotes.com/differences-between-antigen-and-antibody/
• https://www.differencebetween.com/difference-between-antigen-and-vs-antibody/
• https://byjus.com/biology/difference-between-antigen-and-antibody/