General Structure of Antibodies
- Basic Unit
All antibodies have a Y-shaped structure composed of
– Two heavy (H) chains: Longer polypeptide chains.
– Two light (L) chains: Shorter polypeptide chains.
– These chains are connected by disulfide bonds.
– The Fab region (arms of the Y) binds to antigens.
– The Fc region (stem of the Y) interacts with immune cells.
- Variable and Constant Regions:
– Variable region (V): Found at the tips of the Fab region, responsible for antigen specificity.
– Constant region (C): Provides structural stability and immune system interactions.
Structures of Each Antibody Class
- IgG
– Monomeric structure.
– Represents 70-75% of immunoglobulins.
– can be further divided into IgG1,IgG2,IgG3 and IgG4.
-Mw 150000 Da
– The most abundant antibody in the blood and extracellular fluid.
– Functions: Long-term immunity, opsonization, complement activation, and crossing the placenta.
2. IgA
– Found as monomers or dimers (linked by a J-chain).
– Secretory IgA is dimeric and serum mainly IgA is monomer.
– Represents up to 15% of total immunoglobulins.
– Mw 320 000 Da
– Secreted in mucosal surfaces (e.g., saliva, tears, and breast milk).
– Functions: Protects mucosal surfaces by neutralizing pathogens.
3. IgM
– Pentameric structure (five monomers linked by a J-chain).
– First antibody produced during an immune response.
– Represents up to 10% of total immunoglobulins.
-Mw 900 000 Da
– Functions: Strong complement activation and agglutination.
4. IgE
– Monomeric structure.
– Binds to mast cells and basophils.
-Represents only up to 0.01% of total Igs.
-Mw 200 000 Da
– Functions: Involved in allergic reactions and defense against parasitic infections.
5. IgD
– Monomeric structure.
– Found on the surface of immature B cells.
– Represents only up to 0.5% /less of total Igs.
-Mw 180 000 Da.
– Functions: Acts as a receptor for antigen recognition.
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