Antibodies and Vaccination

This section explores the molecular structure of antibodies and how this structure dictates their functions in the immune system. We will also examine the principles behind vaccination and how it leverages antibody production for long-term immunity.

Antibody Structure

Antibodies, also known as immunoglobulins (Ig), are Y-shaped proteins produced by plasma cells in response to an antigen. The structure of an antibody is highly conserved across different classes but exhibits key variations in its functional regions.

An antibody molecule consists of four polypeptide chains: two heavy chains and two light chains. These chains are linked together by disulfide bonds. The antibody structure can be divided into two main regions:

• Fab Region (Fragment antigen-binding): This region is responsible for antigen binding. It consists of the variable (V) domains of the heavy and light chains. Each Fab region has a unique antigen-binding site determined by the amino acid sequence of the V domains.
• Fc Region (Fragment crystallizable): This region is responsible for effector functions, such as binding to immune cells and activating complement. The Fc region is largely constant across different antibody classes.

Antigen Binding and Specificity

The antigen-binding site within the Fab region is formed by the complementarity-determining regions (CDRs) on the V domains. These CDRs make specific contact with the antigen, leading to a highly specific interaction. The shape of the antigen-binding site is complementary to the shape of the antigen, much like a lock and key.

Antibody Classes

There are five main classes of antibodies, each with distinct properties and functions:

Antibody Class	Abbreviation	Major Function	Distribution
IgG	IgG	Most abundant in blood and tissue fluids; provides long-term immunity; crosses the placenta.	Blood, tissue fluids, breast milk
IgM	IgM	First antibody produced during an infection; effective at complement activation.	Blood
IgA	IgA	Found in mucosal secretions (e.g., saliva, tears, breast milk); protects mucosal surfaces.	Mucosal surfaces, breast milk
IgE	IgE	Involved in allergic reactions and parasitic infections.	Bound to mast cells and basophils
IgD	IgD	Function is not fully understood; found on B cells.	B cells

Vaccination

Vaccination is a method of preventing infectious diseases by exposing the body to a weakened or inactive form of a pathogen (or a part of it). This triggers an immune response, leading to the production of antibodies against the pathogen. These antibodies provide protection if the individual is later exposed to the live pathogen.

1. Antigen Exposure: A vaccine contains antigens derived from the pathogen (e.g., proteins, polysaccharides).
2. Immune Response: The antigens are recognized by the immune system, triggering a cascade of events involving B cells and T cells.
3. Antibody Production: B cells differentiate into plasma cells, which produce antibodies specific to the vaccine antigen.
4. Memory Cells: Some B cells and T cells become memory cells, which provide long-term immunity.
5. Secondary Response: If the individual is later exposed to the live pathogen, memory cells rapidly recognize the pathogen and mount a faster and stronger antibody response, preventing or reducing the severity of the infection.

Vaccines can be categorized based on the type of antigen they contain:

• Live-attenuated vaccines: Contain weakened versions of the pathogen.
• Inactivated vaccines: Contain killed pathogens.
• Subunit, recombinant, polysaccharide, and conjugate vaccines: Contain specific antigens from the pathogen.

Relationship between Antibody Structure and Function

The specific structure of an antibody, particularly the amino acid sequence of the CDRs, determines its ability to bind to a specific antigen. The Fc region's structure dictates its interactions with other immune cells, such as macrophages and complement proteins. This intricate relationship between structure and function is crucial for the effectiveness of the antibody response and the success of vaccination.