What are Antibodies? Everything You Need to Know
When you catch a cold or flu, you might notice mild swelling around your neck. This happens because your lymph nodes accumulate white blood cells (WBCs) that help fight off invading germs. Among these WBCs are specialised defenders called antibodies, which play a key role in protecting your body from infections. In this article, we will delve into the structure of antibodies, understand their production, explore the types of antibodies and their functions, and highlight how these tiny Y-shaped proteins help maintain our immunity.
Introduction
Antibodies, also known as immunoglobulins (Ig), are specialised proteins produced by B lymphocytes (a type of white blood cell) in response to the presence of antigens. An antigen is any foreign substance—such as bacteria, viruses, toxins, or other pathogens—that triggers an immune response in the body.
Key Points
Antibodies are Y-shaped proteins designed to identify and neutralise specific antigens.
They circulate in bodily fluids, including blood and lymph, and can also be present in secretions like saliva and breast milk.
Once an antibody binds to its corresponding antigen, it can mark the pathogen for destruction, neutralise its harmful effects, or initiate a cascade of immune responses.
Structure of Antibody Molecule
Understanding the structure of antibody molecules is crucial to appreciate antibody structure and function. Each antibody typically has a Y-shaped configuration made of four polypeptide chains:
Two Heavy (H) Chains: These are longer polypeptide chains that determine the class (isotype) of the immunoglobulin.
Two Light (L) Chains: These are shorter and help form the antigen-binding sites.
Immunoglobulin Structure
When we talk about immunoglobulin structure, the chains are held together by disulphide bonds and other non-covalent interactions. The main regions of an antibody molecule are:
Variable (V) Region: Present at the tips of the Y shape, forming the antigen-binding sites. This region differs from one antibody to another, enabling specificity for unique antigens.
Constant (C) Region: The stem of the Y shape and portions of the arms that do not vary significantly among antibodies of the same class. This region determines how the antibody interacts with other components of the immune system.
Key Functional Segments
Fab (Fragment Antigen-Binding) Region: The two ‘arms’ of the Y that bind specifically to the antigen.
Fc (Fragment Crystallisation) Region: The ‘stem’ that interacts with various immune cells (like macrophages) and complements proteins, thus triggering a wider immune response.
By studying the structure of antibodies, we learn how the molecule locks onto an antigen and instigates its elimination.
Types of Antibodies and Their Functions
One of the most important aspects of immunoglobulin structure is that it allows for different classes (isotypes) of antibodies. Each class has distinct roles and unique properties. Let’s explore the types of antibodies and their functions:
1. IgM
First Responder: IgM is produced first upon initial exposure to an antigen.
Pentameric Form: Usually circulates as five units joined together, making it effective at clumping pathogens.
Functions: Facilitates agglutination (clumping of antigens) and activates the complement system, thus enhancing pathogen destruction.
2. IgG
Most Abundant: Constitutes about 80% of the total antibody content in the blood.
Crosses Placenta: IgG is the only antibody that can cross the placenta, providing foetal immunity.
Functions: Neutralises toxins, supports phagocytosis, and offers long-term protection.
3. IgA
Secretory Antibody: Predominantly found in saliva, tears, breast milk, and intestinal fluids.
Dimeric in Secretions: Often present as two units linked together, especially in bodily secretions.
Functions: Acts as the first line of defence on mucosal surfaces, preventing the attachment of pathogens to epithelial cells.
4. IgD
Receptor on B Cells: Found mainly bound to the surface of B lymphocytes.
Functions: Plays a key role in the activation and regulation of B cells.
5. IgE
Least Abundant: Accounts for a very small fraction of antibodies in circulation.
Allergic Reactions: Responsible for immediate hypersensitivity reactions (e.g., pollen allergies).
Functions: Binds to allergens and triggers histamine release from mast cells, causing inflammation.
When learning about the types of antibodies and their functions, remember that each class is essential in different defence strategies, ensuring a comprehensive immune response.
Antibody Production and Mechanism of Action
Primary Response
When a pathogen enters the body for the first time:
Macrophages or other antigen-presenting cells engulf the pathogen and present antigen fragments on their surface.
Helper T Cells recognise these fragments and activate B Cells.
Plasma B Cells produce antibodies with a specific region (paratope) designed to bind to the pathogen’s unique region (epitope).
Also Read: Difference Between T Cells and B Cells
Secondary Response
Once the infection subsides, some Memory B Cells remain in the body.
If the same pathogen re-enters, these memory cells promptly produce large quantities of antibodies, rapidly neutralising the threat.
This efficient response underscores the importance of antibody structure and function in immune memory, explaining how vaccines work by training the immune system to respond faster.
Difference Between Antigen and Antibody
Understanding the distinction between antigens and antibodies provides clarity on their roles:
Read More: Difference Between Active and Passive Immunity
Additional Insights: Monoclonal vs. Polyclonal Antibodies
To enrich our understanding beyond the basics:
Monoclonal Antibodies
Produced by identical immune cells that are clones of a unique parent cell.
Recognise a single epitope on an antigen.
Widely used in diagnostics (e.g., pregnancy tests) and therapeutics (e.g., targeted cancer therapy).
Polyclonal Antibodies
Generated by multiple B cell clones in the body.
Recognise different epitopes on the same antigen.
Highly useful in research where strong detection of a pathogen or protein is needed.
This added dimension highlights how the structure of the antibody molecule and its generation can be harnessed for advanced medical and research applications.
Clinical Applications and Unique Facts
Antibody Testing: Diagnostic tests (like ELISA) detect specific antibodies against pathogens (e.g., HIV, SARS-CoV-2), confirming exposure or infection.
Therapeutic Antibodies: Monoclonal antibodies are designed to treat diseases such as rheumatoid arthritis, certain cancers, and autoimmune conditions.
ABO Blood Grouping: Antibodies (Anti-A, Anti-B) are central to determining blood groups and ensuring compatibility in blood transfusions.
Swelling of Lymph Nodes: During infections, the body produces more B cells and antibodies, leading to enlarged lymph nodes commonly felt around the neck or underarms.
These unique applications further emphasise why a detailed grasp of antibody structure and function is so valuable in both everyday health and specialised medicine.
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FAQs on Antibodies: Definition, Structure, and Role in Immunity
1. What is the study of the interaction between antigen and antibody called?
The study of interactions between antigens and antibodies in blood and other bodily fluids is called serology.
2. How many classes of antibodies are there in humans?
There are five main classes (isotypes) of antibodies—IgG, IgM, IgA, IgD, and IgE. Each has a unique immunoglobulin structure and function in the immune system.
3. Which antibody is transferred from the mother to the foetus?
IgG is the only class that can cross the placenta, providing crucial immunity to the developing foetus.
4. Why is the structure of the antibody molecule often described as Y-shaped?
The Y-shaped arrangement is due to the presence of two heavy chains and two light chains. This structure of antibody molecule allows two antigen-binding sites at the ‘arms’ and an Fc region at the ‘stem’ for interaction with immune cells.
5. Can the immune system remember pathogens?
Yes. Memory B Cells remain in circulation after an infection subsides, allowing the immune system to recognise and respond faster if the same pathogen re-enters the body.
