Antibodies (immunoglobulins, Ig) are Y-shaped proteins produced by B cells that bind precisely to specific antigenic determinants (epitopes) on pathogens. Understanding antibody structure, classes, and mechanisms of action provides a foundation for vaccine design and immunotherapy.
Basic Knowledge
Antibody structure: A Y-shaped tetramer consisting of two heavy chains (H chains) and two light chains (L chains) linked by disulfide bonds. The variable regions (V regions) at the tips bind antigens (the Fab regions), while the constant region (Fc region) at the base performs effector functions.
Antibody classes (isotypes): IgM = early immune response, pentamer, complement activation. IgG = main serum antibody, secondary response, can cross the placenta. IgA = abundant on mucosal surfaces and in breast milk, dimer. IgE = involved in allergies and parasitic infections; sensitizes mast cells. IgD = B-cell receptor.
Mechanisms of antibody action: ①Neutralization = masks the surface of a pathogen with antibodies and blocks receptor binding or inactivates toxins. ②Opsonization = antibody-coated pathogens are more easily phagocytosed by macrophages and neutrophils carrying Fc receptors. ③Complement activation = IgM and IgG initiate the complement cascade, dissolving pathogen membranes (MAC formation).
Clonal selection theory: B cells with diverse antigen receptors already exist, and a specific antigen selects and proliferates the corresponding clone. This is the basis of B-cell diversity. Somatic hypermutation and class switching (isotype switching) change the precision and class of the response.
Compare IgG and IgM in terms of structure, timing of production, and function.
Answer: ① Structure: IgG is a monomer (tetramer), whereas IgM is a pentamer with a larger molecular mass. ② Timing of production: IgM is produced first during the early stage of infection (the primary response), while IgG is produced later through class switching and is the main antibody in the secondary response. ③ Function: IgM is highly effective at activating complement. IgG can cross the placenta and provide passive immunity to the fetus, and it performs multiple functions including opsonization and complement activation.
- Antibody = Y-shaped tetramer of two heavy chains + two light chains
- Fab region = antigen-binding site (variable region)
- Fc region = effector functions (complement and interaction with phagocytes)
- IgM = early response, pentamer, complement activation
- IgG = main serum antibody, secondary response, crosses the placenta
- IgE = allergy and mast-cell sensitization
- Antibody actions = neutralization, opsonization, and complement activation
Points to Note
① Antibody diversity is generated at the genetic level during B-cell differentiation through V(D)J recombination. ② When IgE binds to mast cells and the body is exposed to the same antigen again, histamine is released and an allergic reaction occurs. ③ Monoclonal antibodies are antibodies from a single clone that target one type of antigen epitope and are widely used as diagnostic and therapeutic drugs.
Practice
- Explain antibody structure using the terms Fab region, Fc region, heavy chain, and light chain.
- What is opsonization? Explain it including its relationship to phagocytosis.
- What immune response involves IgE? Describe the mechanism by which it causes an allergy.