Antibodies: Structures, functions and diversity

STRUCTURE

Antibodies, also known as immunoglobulins, are Y-shaped proteins produced by B cells of the immune system. They play a critical role in the immune response by recognizing and neutralizing specific pathogens, such as bacteria and viruses. Antibodies have a well-defined structure that allows them to perform their functions effectively. Let's break down the structure of antibodies:

BASIC STRUCTURE OF ANTIBODIES

• Y-Shaped Molecule: Antibodies have a Y-shaped structure, often depicted as two "arms" and a "stalk." This Y-shape is formed by four polypeptide chains.
• Four Polypeptide Chains: Antibodies consist of two identical heavy chains (H) and two identical light chains (L). These chains are held together by disulfide bonds and are organized into two identical arms.
• Variable and Constant Regions: Each antibody has two main regions in its structure: the variable region and the constant region.
• Variable Region: The tips of the Y-shaped arms contain the variable regions of both heavy and light chains. These variable regions are highly variable in sequence and are responsible for binding to specific antigens (the target molecules on pathogens).
• Constant Region: The base of the Y-shaped molecule contains the constant regions of the heavy chains. The constant region is more uniform in structure and determines the class or isotype of the antibody (e.g., IgG, IgA, IgM).

ANTIGEN BINDING SITE

• The tips of the Y-shaped antibody structure, formed by the variable regions of both heavy and light chains, create the antigen-binding site. This site is highly specific and can recognize and bind to a particular antigen, like a lock fitting a key.

 

FUNCTIONS

• Neutralization: Antibodies can block the harmful effects of pathogens by binding to them, preventing them from infecting host cells. This is known as neutralization.
• Opsonization: Antibodies can tag pathogens for destruction by other immune cells, such as phagocytes (cells that "eat" invaders).
• Agglutination: Antibodies can cause pathogens to clump together, making it easier for immune cells to locate and eliminate them.
• Activation of Complement: Antibodies can trigger the complement system, a group of proteins that leads to the destruction of pathogens through a cascade of reactions.
• Memory: Some antibodies can be produced in larger quantities during a secondary immune response, thanks to memory B cells, allowing for faster and more effective defence if the same pathogen is encountered again.

The structure of antibodies is highly adaptable and specific, which enables them to recognize and combat a wide variety of pathogens. Their ability to bind to antigens is a fundamental aspect of the immune response, and it plays a central role in our immune system's ability to protect us from infections.

DIVERSITY

 

Sl No	Antibody	Heavy Chain			Light Chain	Molecular Formula
		Types	Amino Acid	Molecular Weight(kDa)
1	Ig A	α	446	55-60	κ or λ	(α2κ2)n or (α2λ2)n
2	Ig G	γ	472	50-60	κ or λ	γ2κ2 or γ2λ2
3	Ig E	ε	550	70-73	κ or λ	ε2κ2 or ε2λ2
4	Ig H	μ	576	70	κ or λ	μ2κ2 or μ2λ2
5	Ig D	δ	-	62	κ or λ	δ2κ2 or δ2λ2

Here's a table summarizing the five main types of antibodies (immunoglobulins) and their structures and functions:

Antibody Type	Structure	Functions
IgG	- Two heavy chains - Two light chains - Monomeric (single Y-shaped unit)	- Most common in the bloodstream - Fights bacterial and viral infections - Provides long-term immunity
IgA	- Dimers or multimers with "J chain" - May have a "secretory component"	- Predominant in mucosal areas (respiratory, digestive tracts) - Defends against pathogens at mucosal surfaces - Found in secretions like saliva, tears, and breast milk, providing passive immunity to infants
IgM	- Pentameric (five Y-shaped units) - Ten antigen-binding sites	- Often the first antibodies produced in response to an infection - Efficiently agglutinates (clumps) pathogens - Initiates the immune response as an early defence mechanism
IgE	- Two heavy chains - Two light chains	- Primarily involved in allergic responses - Binds to allergens, triggering histamine release and allergic symptoms - Also associated with defence against parasitic infections
IgD	- Monomeric (single Y-shaped unit)	- Found on the surface of B cells, not fully understood - May play a role in B cell activation

The diversity of antibodies is a testament to the complexity of the immune system.

• Gene Rearrangement:
• Our bodies have a limited set of antibody genes, but through gene rearrangement, they can generate an enormous variety of antibodies.
• Somatic Hypermutation:
• B cells, which produce antibodies, undergo somatic hypermutation, introducing random changes in the antibody genes. This process creates even more diversity.
• Isotypes and Subtypes:
• Antibodies come in different classes (isotypes) such as IgG, IgM, IgA, IgD, and IgE, each with unique functions.
• Isotypes have subtypes, further adding to the diversity. For example, IgG has subclasses like IgG1, IgG2, etc.

Conclusion: The Versatile Protectors

In conclusion, antibodies are like versatile superheroes, each equipped with unique powers and specialities. From recognizing invaders to rallying immune cells, their diversity and precision are the keys to our immune system's success. As we journey through immunology, keep in mind that the study of antibodies continues to unravel new mysteries, offering insights into how our bodies fend off diverse threats.

Stay curious, for the story of antibodies is one of complexity, adaptability, and the relentless pursuit of protection against the ever-changing landscape of pathogens.