Structure of an Antibody Molecule – Class 12 | Chapter – 8 | Biology Short Notes Series PDF

Structure of an Antibody Molecule: An antibody, also known as an immunoglobulin, is a Y-shaped structure which consists of four polypeptides — two heavy chains and two light chains. This structure allows antibody molecules to carry out their dual functions: antigen binding and biological activity mediation. Each function is carried out by different parts of the antibody: fragment antigen-binding (Fab fragment) and fragment crystallizable region (Fc region).

Diagram of Antibody Molecule

Types of Antibody Molecule

• IgG antibody: Immunoglobulin G (IgG) antibodies are large globular proteins with a molecular weight of about 150 kDa made of four peptide chains. It contains two identical γ (gamma) heavy chains of about 50 kDa and two identical light chains of about 25 kDa, thus a tetrameric quaternary structure.
• IgM antibody: Immunoglobulin M (IgM) antibodies are constructed of five or six units (i.e. mostly as pentamers but also hexamers occur) which are each comprised of two heavy-chains (μ-chains) and two light chains, bound together by disulfide bonds and a so-called J-chain.
• IgA antibody: Immunoglobulin A (IgA) antibodies consist of heavy (H) and light (L) chains. Each H chain is comprised of the constant region (Cα1, Cα2, Cα3), hinge region and the Variable (V) region. Light chains consist of the CL and Vκ or Vλ elements.
• IgE antibody: Immunoglobulin E (IgE) antibodies have only been found in mammals. IgE is synthesised by plasma cells. Monomers of IgE consist of two heavy chains (ε chain) and two light chains, with the ε chain containing 4 Ig-like constant domains (Cε1-Cε4).
• IgD antibody: Immunoglobulin D (IgD) antibodies are expressed in the plasma membranes of immature B-lymphocytes. IgD is also produced in a secreted form that is found in small amounts in blood serum. IgD plays a role in the induction of antibody production.

Functions of Antibody Molecule

• Binds to pathogens

• Activates the immune system in case of bacterial pathogens

• Directly attacks viral pathogens

• Assists in phagocytosis

• Antibody provides long-term protection against pathogens because it persists for years after the presence of the antigen.

• It neutralizes the bacterial toxins and binds the antigen to enhance its efficiency.

• They also act as the first line of defence for mucosal surfaces.

• They ingest cells by phagocytosis.

• Few antibodies bind the antigen present on the pathogens. These aggregates the pathogen and they remain in secretions. When the secretion is expelled out, the antigen is also expelled.

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By Team Learning Mantras