Definition
Immunoglobulins (Ig), also called antibodies, are glycoproteins produced by plasma cells (differentiated B cells).
They are specific for particular antigens.
Main Components
Heavy Chains (H chains) – Large polypeptide chains (2 per antibody).
Light Chains (L chains) – Smaller polypeptide chains (2 per antibody).
Variable Region (V region) – Binds to the antigen (Fab region).
Constant Region (C region) – Determines antibody class (Fc region).
Hinge Region – Provides flexibility for antigen binding.
Major Immunoglobulin Classes (Isotypes)
1) IgM
First antibody produced in a primary immune response.
Exists as a pentamer (5 monomers joined by a J chain) in circulation → high avidity.
Very efficient at activating the complement system.
2) IgG
Most abundant immunoglobulin in serum.
Four subclasses (IgG1, IgG2, IgG3, IgG4).
Crosses the placenta (provides passive immunity to the fetus).
Good at opsonization, neutralization, and complement activation.
3) IgA
Found mainly in mucosal areas (secretions like tears, saliva, colostrum, and secretions in the gut).
Typically, a dimer (2 monomers joined by a J chain) in secretions; monomer in serum.
Important for mucosal immunity.
4) IgE
Binds with high affinity to Fc receptors on mast cells and basophils.
Plays a major role in allergic reactions (Type I hypersensitivity) and defense against parasites.
5) IgD
Found primarily on the surface of naïve B cells.
Role in B-cell activation is not fully understood but is important in the early stages of the immune response.
