Antibody - Structure, Functions, and Types
Last Updated : 21 Feb, 2024
Antibody proteins are Y-shaped proteins that recognize and neutralize pathogens in the immune system. Antibodies play an important role in immune defense. Antibody structure consists of four polypeptide chains—two heavy and two light—and they form a distinctive structure with variable and constant regions.
There are five main types of antibodies: IgM, IgG, IgA, IgD, and IgE, each having a specific role in the immune responses. Antibody functions include recognizing and binding to specific antigens, marking them for destruction or neutralization by the immune system. In this article, we will study antibody structure, function, types, and differences between antibodies and antigens.
What are Antibodies?
An antibody (Ab) is a large, Y-shaped protein that the immune system utilizes to recognize and destroy foreign substances including harmful bacteria and viruses. The antigen, or distinctive molecule of the infection, is recognized by the antibody. These two structures can bind together precisely because each Y-shaped tip of an antibody has a paratope (like a lock) that is specific for one particular epitope (like a key) on an antigen.
A microbe or an infected cell can be directly neutralized by an antibody using this binding process, or it can be marked for attack by other immune system components, for instance, by inhibiting a component of a virus that is necessary for its invasion. Antibodies are essential components of vaccinations, aiding the body in developing immunity to specific diseases.
Structure of the Antibody
Antibodies, also known as immunoglobulins (Ig), have a Y-shaped structure. In detail, the antibody molecule structure is described below:
- Each arm of the Y consists of two types of protein chains—heavy chains and light chains.
- Varaible regions are present on that tip of the Y-shaped arms. It recognize and bind to specific antigens.
- Constant region are present on the lower part of the Y shaped arm, including the stem. It determine the class of the antibody (IgA, IgD, IgE, IgG, or IgM).
- Antigen binds at the tip of the Y shape antibody. The region where it binds is called the antigen-binding site.
- The light and heavy chain binds together through disulfide bonds. It help in maintaining the structural integrity of antibodies.
- The stem of the Y is also known as the Fc region. It is involved in various immune responses, such as binding to cell receptors or activating complement proteins.
Antibody Diagram
The well-labeled diagram of antibody is shown below:
Antibody Types
There are 5 types of antibodies based on antibody structure and function: IgG, IgM, IgA, IgD, and IgE. They are spread throughout the body and have several functions.
IgG
IgG is a Y-shaped molecule with two antigen-binding sites.
- It is the primary antibody in the blood.
- It has a strong affinity for binding to bacteria and toxins.
- It is important to the biological defense system.
- IgG from the mother's body protects a baby since it is the only isotype that can cross the placenta.
IgM
IgM is pentameric structure with ten antigen-binding sites
- IgM is mostly found in the blood and is made up of five basic Y-shaped unit cells.
- After pathogen invasion it is the first to be produced.
- It provides early response to infections.
IgA
IgA has monomeric form in blood, dimeric form with two Y-shaped molecules linked by a J chain in mucosal secretions.
- It also forms dimers, or molecules with two Ys, in secretions like intestinal fluid, nasal discharge, and saliva to thwart bacterial invasion from mucosal membranes.
- Additionally, found in breast milk, it shields a newborn's digestive system from viral and bacterial illness.
IgD
IgD is a monomeric molecule similar to IgG.
- It is found on the surface of B cells.
- IgD acts as a membrane receptor.
- It aids in the stimulation of antibody synthesis.
- It also helps in the protection of respiratory tract infections.
IgE
IgE shows monomeric structure.
- IgE was thought to have its origins in immune responses to parasites.
- It plays a role in allergies like pollinosis by attaching to mast cells.
- It triggers the release of histamines from mast cells and basophils.
Antibodies Functions
Following are some of the functions of antibodies:
- Antibodies recognise and bind to specific anigen.
- It blocks the toxins and harmful effects of pathogen by neutralizing it.
- It marks the pathogen for destruction by immune cells.
- Antibodies trigger the complement system for pathogen destruction.
- It stimulate natural killer cells to destroy infected cells.
- It forms memory for rapid and robust response upon re-exposure to the same pathogen.
- It regulate the various immune responses.
Production and Mechanism of Action
The immune system can identify foreign substances when they enter the body because the antigen's surface molecules are different from those found in the body. The immune system uses a number of methods, including one of the most crucial ones, antibody production, to get rid of the pathogen.
B lymphocytes, often known as B cells, are specialized white blood cells that produce antibodies. A clone is a collection of identical cells that are created when a B cell divides and matures in response to an antigen binding to the B cell surface. Millions of antibodies are secreted into the lymphatic and circulation by mature B cells, also known as plasma cells.
Mechanism of Action of Antibodies
Mechanism of action of the antibodies is as follows:
- Antibodies recognize foreign substance or pathogen and bind to antigens.
- Antibodies neutralize the activity or function of pathogens by binding to them. It prevent pathogen from interacting with host cells.
- Antibodies increases phagocytosis by binding to pathogens. It mark them for destruction by phagocytic cells such as macrophages and neutrophils.
- Antibodies can destroy the pathogens through different processes like:
- Cause the aggregation of pathogens that make it easier for phagocytes to engulf and eliminate pathogens.
- Cause soluble antigens to become insoluble. It forms immune complexes that are more easily removed by phagocytic cells.
- It cross-link multiple antigens that result in the formation of large complexes that are easily removedby immune cells.
- Can stimulate NK cells to release cytotoxic substances. It results in elimination of infected or abnormal cells.
- Antibodies play an important role immunological memory. When the same antigen enters the body the memory B cells quickly produce a antibody response. It provides a faster and more effective immune reaction.
Antibodies shows various mode of actions to eliminate, neutralize, and regulate immune responses. It contribute to the body's defense against pathogens and foreign substances.
Applications of Antibodies in Medicines
In medicine, scientists have found many ways to use these antibodies to help people stay healthy:
- Treating Diseases: Antibodies can be used as medicines to target and destroy specific harmful substances in the body, such as cancer cells or disease-causing bacteria.
- Vaccines: Vaccines contain weakened or harmless versions of viruses or bacteria, which help our bodies produce antibodies to fight off those germs. This helps us build immunity against diseases like measles or the flu.
- Diagnostic Tests: Antibodies can also be used in tests to detect if someone has a particular disease. For example, in COVID-19 tests, antibodies help identify if a person has been infected with the virus in the past.
- Targeted Therapy: Some medicines use antibodies to specifically target certain cells in the body, like cancer cells, while leaving healthy cells unharmed. This helps reduce side effects and make treatments more effective.
Difference between Antigen and Antibody
Antigens and Antibodies can be differentiated by the following:
| Feature | Antigen | Antibody |
|---|
| Definition | A substance that triggers an immune response and can stimulate the production of antibodies. | Proteins produced by the immune system in response to the presence of an antigen. |
| Origin | Can be derived from pathogens (bacteria, viruses), foreign substances, or even self-substances in certain autoimmune conditions. | Produced by B cells (a type of white blood cell) in response to the presence of antigens. |
| Nature | Antigens can be proteins, carbohydrates, lipids, or nucleic acids. | Antibodies are proteins (immunoglobulins) with specific structures that recognize and bind to antigens. |
| Function | Initiates and elicits an immune response. | Acts against antigens by binding to them and neutralizing or marking them for destruction by other immune cells. |
| Recognition | Recognized by the immune system as foreign or non-self. | Specifically binds to the complementary antigen that triggered its production. |
| Diversity | Diverse; various antigens can elicit an immune response. | Highly diverse; numerous types of antibodies, each specific to a particular antigen. |
| Specificity | Generally not highly specific; the immune system can recognize a broad range of antigens. | Highly specific; each antibody is designed to recognize and bind to a specific antigen. |
| Production | Produced by the body or introduced through vaccination. | Produced by B cells through a complex process called somatic recombination. |
| Role in Immunity | Initiates the immune response by triggering the production of antibodies. | Essential for the immune system's ability to recognize, neutralize, and eliminate pathogens or foreign substances. |
| Examples | Pathogen proteins, toxins, allergens. | IgG, IgM, IgA, IgD, IgE (classes of antibodies). |
| Size and Structure | Variable depending on the type of antigen. | Y-shaped structure; composed of two heavy chains and two light chains. |
| Half-life in Blood | Antigens may persist for a shorter duration. | Antibodies can circulate in the bloodstream for an extended period |
Conclusion: Antibody - Structure, Functions, and Types
Antibodies are proteins produced by the immune system to neutralize harmful pathogens and protect the body from infections. The antibody structure consists of four polypeptide chains: two heavy chains and two light chains, forming a Y-shaped molecule with variable and constant regions. There are 5 types of antibodies, each with a different function. Antibody functions to protect and maintain our health. This is why studying the antibodies structure and function is important for understanding our immuhne system.
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