Antibodies

When disease-causing germs gain entry into the blood stream, they produce poisonous substances called toxins (meaning poisons). In response, the lymphocytes of the blood produce chemical substances called antibodies which circulate freely in the blood plasma.

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The B-lymphocytes produce an army of proteins in response to pathogens in our blood to fight with them. These proteins are called antibodies.

Each antibody is a Y-shaped molecule composed of two heavy (H) chains and two light (L) chains, linked by disulphide bonds.

Each chain has two regions:

• Variable (V) region: antigen-binding site that differs between antibodies.
• Constant (C) region determines antibody class and stays consistent within a type.

Additional stabilisation comes from non-covalent interactions between chains.

Structure of Antibodies

A-F. Structure of different types of immunoglobulin antibodies: A. IgG; B. IgM; C. IgA (monomeric); D. IgA (dimeric); E. IgD; F. IgE.

• Precipitation: Converts soluble antigens into insoluble complexes for removal.
• Agglutination: Clumps bacteria or antigen-bearing cells (e.g., blood typing reactions).
• Complement Fixation: Activates complement proteins, leading to pathogen lysis.
• Opsonisation: Coats pathogens, marking them for phagocytosis by macrophages.
• Secretory Protection (IgA): Passive immunity transfer via colostrum/breast milk.
• Hypersensitivity (IgE): Triggers allergic/type I hypersensitivity reactions.

• Antibodies are specialised defensive proteins produced by B-lymphocytes to protect the body against disease-causing pathogens and their toxins.
• They feature a distinct Y-shaped molecular structure consisting of two heavy chains and two light chains connected by disulphide bonds.
• Each antibody contains a variable region designed to bind to specific antigens and a constant region that determines its overall classification.
• These proteins are categorised into five primary classes – IgG, IgM, IgA, IgD, and IgE – each fulfilling a specific role within the immune system.
• Their specific functions vary widely, such as IgG providing passive immunity to foetuses, IgA protecting mucosal surfaces, and IgE mediating allergic reactions.
• Antibodies neutralise systemic threats through diverse mechanisms, including clumping pathogens together, marking them for cellular destruction, and activating further immune responses.