Antibodies (Immunoglobulins): Definition and Basic Structure
Antibodies (AB), also known as immunoglobulins (Ig), are large, Y-shaped glycoproteins.
- They are produced by plasma cells, which are differentiated B cells
- Antibodies are used by the immune system to neutralise pathogens such as bacteria and viruses.
- They are part of the humoral immune system.
Immunoglobulin Structure: Chains, Regions, Domains
- Each antibody consists of four polypeptide chains: two identical heavy chains (H) and two identical light chains (L) .
- These chains are connected by disulfide bonds.
- The light chains appear as two forms: kappa (κ) or lambda (λ)
- The heavy chains determine the antibody class (isotype) and biological There are five main heavy chain variants: α, δ, ε, γ, and μ, corresponding to IgA, IgD, IgE, IgG, and IgM respectively.
- Antibodies have structural domains. In monomeric IgG, each chain has domains of ~110 amino acids; light chains have two domains, heavy chains have four or five. Immunoglobulins, MHC class I, and MHC class II molecules all contain comparable Ig domains.
- The Variable Region is the part of the antibody that binds specifically to the antigen It is different for each antibody clone .This region includes the "tip" of the Y and consists of highly variable amino acids (110-130 amino acids are involved in specific binding, or 12-15 amino acids contact the peptide region in the antigen) giving the antibody its specific bonding capabilities. The specificity is determined by the amino acid sequence of the variable domains of the heavy and light chains. Diversity in this region (Idiotypes) is ensured by genetic recombination of V, D, and J segments.
- The Antigen-Binding Site (ABS) or Paratope is the part of the antibody molecule that contacts the epitope on the antigen. A monomeric immunoglobulin has two identical ABS, forming the ends of the two short arms of the 'Y'.
- The Fab region (Fragment Antigen-Binding) consists of the variable region and part of the constant region of both heavy and light chains; it is the part that recognises and binds the antigen.
- The Constant Region is the part of the antibody that is the same in all antibodies of a given class .This region, the "trunk" of the Y, mediates immune effector functions.
- The Fc region (Fragment Crystallisable) is made up of the constant domains of the heavy chains. It mediates the antibody's ability to communicate with and activate other components of the immune system and determines the antibody's class effect.