Immunology Class 8: Antibodies and Antigens

Antibodies and the Immune System

• Antibodies are part of the adaptive immune system.
• They serve as antigen recognition molecules along with B cells (antibody/immunoglobulin), T cells (T-cell receptor), and all cells (Major Histocompatibility Complex).

Antigens

• Antigens are molecules or epitopes that trigger antibody production and specific receptors on T and B cells, typically derived from foreign materials such as proteins and sugars.

Immunoglobulins Overview

• Immunoglobulin refers to all antibody molecules produced by B cells, which can exist in soluble or membrane-bound forms.
• They recognize specific epitopes on proteins, polysaccharides, lipids, or nucleic acids.
• Genetic rearrangement allows for the potential generation of over 10^9 distinct specificities.

Immunoglobulin Structure

• Immunoglobulins have a Y-shaped structure consisting of two light chains and two heavy chains containing immunoglobulin (Ig) domains.
• Each immunoglobulin molecule has a variable region and constant regions, contributing to their unique functions.

Fab and Fc Components

• The Fab (fragment antigen-binding) component has arms that adapt to bind a large number of antigens.
• The Fc (fragment crystallizable) component determines the biological activities of the antibody.

Immunoglobulin Isotypes

• There are five immunoglobulin isotypes, determined by the heavy chain type.
• The typical concentration hierarchy in human serum is: IgG > IgA > IgM > IgE > IgD (acronym: GAMED).
• IgD is unique as it is never soluble and is always membrane-bound.

Light and Heavy Chains

• There are two classes of light chains: kappa and lambda; they are identical in a single antibody (either 2 kappa or 2 lambda).
• Light chains can associate with any heavy chain, contrasting with five distinct heavy chain classes.

Variable Region and Antigen Binding

• Each chain's amino-terminus contains a variable region that forms the antigen-binding site, while the remaining domains are constant.
• The Fab fragment is essential for antigen binding and consists of both variable and constant domains.

Disulfide Bonds and Stability

• Intramolecular disulfide bonds in antibody structure help form Ig domains.
• Intermolecular disulfide bonds connect heavy and light chains, playing a crucial role in forming the hinge region.

Hypervariable Regions

• Hypervariable regions, or complementary determining regions (CDRs), are critical for specificity in antigen binding, especially CDR3, which shows the greatest variability.

Immunoglobulin Isotype Determinants

• The immunoglobulin isotype is determined by the primary amino acid sequence of the heavy chains.

Idiotypic Differences

• Differences among immunoglobulins are idiotypic, based on antigenic characteristics of the variable regions.

Comparing Isotypic and Idiotypic Differences

• Isotypic differences depend on the heavy chain type (e.g., IgG vs. IgA), while idiotypic differences are based on the variable regions of the chains (e.g., IgG1 vs. IgG2).

Major Immunoglobulin in Serum

• IgG is the predominant immunoglobulin isotype present in serum and plays key roles in immune responses.
• IgG activates the classical complement pathway, acts as an opsonin, is vital for secondary immune responses, crosses the placenta for neonatal immunity, and is crucial in antibody-dependent cell-mediated cytotoxicity.

Test your knowledge on antibodies and their role in the immune system with flashcards focused on immunoglobulin classes and antigen recognition molecules. This quiz covers key concepts that are vital for understanding adaptive immunity. Perfect for students of immunology or related fields!