Antibody: Structure, Types, Functions microbiologystudy

Antibodies are the proteins produced by the B-lymphocytes in response to the antigen or foreign bodies.

• They are also known as immunoglobulins (Ig).
• They are found in the serum fraction of the blood and produced by plasma cells. Approximately 20% of the plasma protein is immunoglobulins.
• Antibodies can travel throughout the body to find and bind to their target antigens (foreign substances).
• The molecular weight of antibodies are generally ranges between 150000-900000 Da.

Structure of Antibody

IgG is the most studied immunoglobulin and serves as a model of the basic structural unit of all antibodies.

The molecule of IgG is Y-shaped and consists of 4-polypeptide chains; two identical heavy (H) chains and two identical light (L) chains which are connected by a disulfide bond.

Different parts of an antibody molecule are described below:

Heavy and Light Chains

• There are 4 peptide chains in an antibody molecule.
• Two long chains called heavy chains and two long chains called light chains.
• Each heavy chain contains more than 400 amino acids, while the light chain usually has 200 amino acids.
• In mammals, H chains are α, δ, ε, γ, and μ types, but L chains are two types known as lambda (λ) and kappa (κ).

Disulfide Bonds and Hinge Region

• Each light chain is linked to a heavy chain by a disulfide bond.
• There are also two disulfide bonds connecting the two heavy chains.
• This part of the antibody is called the hinge region, which functions in maintaining the flexibility of the antibody. Here, the arms of the antibody can bend and form a Y-shaped structure.
• There are also intrachain disulfide bonds in both light and heavy chains.

Constant and Variable Regions

• Intrachain disulfide bonds form loops.
• Each loop is called a domain.
• The light chain contains a single variable domain (V_L)and a single constant domain (C_L), while the heavy chain contains a single variable domain (V_H) and three or four constant domains (C_H).
• Constant domains from the constant region and variable domains form the variable region in each of the heavy and light chains.

Fragment Antigen Binding (Fab) and Fragment Crystalizable (Fc)

• Two identical fragments of the Y-shaped structure of the antibody molecule function for antigen binding sites. This is known as the fragment antigen binding (Fab).
• The antigen binding sites bind to specific antigens in a lock and key pattern and form an antigen-antibody complex.
• The tail section of the antibody is incapable of binding antigen and can be crystallized. This is called fragment crystallizable (Fc).