Antibodies are antigen binding proteins present on the B-cell membrane and secreted by plasma cells. These are specialized serum proteins which are formed in response to the antigen and react specifically to that antigen.
They are serum proteins secret by plasma cells, formed by the activation and differentiation of B-lymphocytes. Recognition of antigen by surface antibodies (receptors) activates B-cells which differentiate into memory and plasma cells. Plasma cells sectrete soluble antibody molecule.
Antibodies are glycoproteins produced in membrane-bound or secreted form by B lymphocytes in response to exposure to foreign structures known as antigens. Chemically, they are globulins. Thus they are termed “immunoglobulins” i.e. Globulins with immune functions.
These serum immunoglobulins comprise about 20-25 % of total serum proteins.
- Antibody molecule is heterodimer of two identical light chain (L) and two identical heavy chains (H). Light chain is small polypeptide of about 25,000 Da while heavy chain is larger polypeptide of 50,000 Da.
- Each light chain is bound to heavy chain with disulphide linkages and covalent interactions (H-bonding, hydrophobic interaction etc). The heavy chain are joined with each other by disulphide linkages and non-covalent interactions to form heterodimer structure.
- About 100-110 amino acid sequence in the amino terminus region of both heavy and light chain have hyper variable region. Variation of this site forms specific binding of antibody with specific antigen. Remaining part of both chains are constant (i.e. Carboxy terminus region is constant).
- Light chain are of two types which are kappa (k) and lamda (⅄) whereas, there are 5 types of heavy chain namely, Gamma (γ), Meu(μ), Alpha (α), Delta (δ) and Epsilon (ε).
- The intra chain disulphide bond within heavy and light chain of antibody forms globular structure called domain. Domains are named on the basis of chain and the regions they are present.
- Light Chain have two domains: Variable light (VL) domain and Constant light (CL) domain.
- Heavy chain have 4 to 5 domains depending upon type of antibody: Variable heavy (VH) domain and Constant heavy (CH-1to 4). Immunoglobulins with heavy chain Meu (μ) and Epsilon (ε) have CH-4 domain.
- VL and VH domains form antigen binding site.
- Immunoglobulin molecule has hinge region in the middle of the molecule. On enzymatic digestion by Papain enzyme 2 fragments of Fab (portion that binds antigen) and 1 fragment of FC (portion of constant region) is formed. However, on recovery only of Fab fragments are obtained while FC portion is digested into numerous small peptides.
Types of Antibodies
The human immunoglobulins are a family of proteins that confer humoral immunity and perform vital roles in promoting cellular immunity. There are five classes of antibodies or immunoglobulins which are IgG, IgM, IgA, IgD and IgE. All these classes have the basic four – chain antibody structure but they differ in their heavy chains termed γ, μ, α, δ and ε respectively.
- Immunoglobulin G (IgG)
- Immunoglobulin M (IgM)
- Immunoglobulin A (IgA)
- Immunoglobulin E (IgE)
- Immunoglobulin D (IgD)
The antibodies are the gamma globulins. Antibodies are often referred to as “first line of defense” against infection. The most important function of antibodies is to confer protection against microbial pathogens. Antibodies confer protection in the following ways:
- They prevent attachment of microbes to mucosal surfaces of the host.
- They reduce virulence of microbes by neutralizing toxins and viruses.
- They facilitate phagocytosis by opsonization of microbes.
- They activate complement, leading to complement-mediated activities against microbes.