|Different types of Immunoglobulins|
- A divalent with two identical antigen binding site.
- 4 subclasses based on antigenic differences in H –chain and number and location of di-sulphide bonds namely IgG1-IgG4
- IgG1 accounts 65% of total IgG
- IgG2 is directed against polysaccharide antigens and forms an important host defence against encapsulated bacteria.
- Most abundant antibody in blood, intestine and lymph
- Predominant antibody in the secondary immune response
- The only antibody with the ability to cross placenta. Here, Fc portion binds to the cell surface receptors of placental cells. Thus the most abundant immunoglobulin in new-borns.
- IgG can activate complement (The other one is IgM)
- Promotes opsonisation or enhances phagocytosis. Phagocytes surface has receptors (ϒ-H chain ) for IgG.
- Consists of H2L2 units (similar to IgG) plus one molecule each of j-joining chain and secretory component. In serum, IgA exists as monomeric H2L2.
- The secretory component is a polypeptide synthesized by epithelial cells that assist IgA passage to the mucosal surface. It also protects IgA from degradation in the intestinal tract.
- Main immunoglobulin in secretions such as colostrum, saliva, tears, respiratory, intestinal and genital tract secretions.
- Prevents attachment of microbes to the mucous membrane
- Does not fix complement
- In serum, it is a pentamer composed of 5-H2L2 units + one molecule of J chain (joining chain).
- IgM is present as a monomer on the B cell surface where it functions as antigen binding receptor
- Pentamer has 10 antigen binding sites
- First immunoglobulin to reach the site of infection that is the major immunoglobulin produced early in the primary response.
- Most efficient in agglutination, complement fixation and other antibody interactions as IgM has 10 antigen binding sites.
- Important in defence against bacteria and viruses
- It functions (monomers) as antigen binding receptors on B cell surface.
- Monomeric protein
- May function as antigen receptor on B cell surface
- Present in small amount in serum
- Least abundant
- Mediates anaphylactic hypersensitivity or allergic reactions
- Major host defense against parasitic infection especially against Helminths (worm infection).
- Mostly present in mucous membrane, skin and lungs
- Does not cross the placenta or fix complement.