IMMUNOGLOBULINS

IMMUNOGLOBULIN G (IgG):  IgG contains two heavy chains and two light chains; heavy chains being of gamma type. Due to its sedimentation coefficient, it is sometimes referred to as 7S lg.  IgG is the major antibody; it constitutes about 75-80% of total immunoglobulins in circulation.  It is the antibody seen in secondary immuno response.  It can pass from vascular compartment to interstitial space. It can cross-placental barrier, and protects the newborn child from infections. These maternal antibodies are seen in neonatal circulation up to 2-4 months of neonatal age.  Placental crossing of IgG also explains the Rh iso-immunization. This occurs when mother is Rh-negative, and fetus is Rh-positive, and when ABO system antigens are similar to both mother and fetus.

During parturition, fetal RBCs may enter into maternal circulation, leading to formation of anti-Rh antibodies.  During next pregnancy, these antibodies, being IgG in class, can enter into fetal circulation, causing fetal hemolysis, neonatal jaundice, and in severe cases, neonatal death or miscarriage. Passively injected anti-Rh antibody, when injected within 24 hrs of delivery of first child, will avert the isoimmunization and can protect future pregnancy.

IMMUNOGLOBULIN M (IgM):  IgM are macroglobulins or 19S immunoglobulins.  Five subunits, each having 4 peptide chains (total 10 heavy chains and 10 light chains) are joined together by a J-chain polypeptide. It can combine with 5 antigens simultaneously, and so IgM is very effective for agglutinating bacteria.  Being a large molecule, it cannot come out of vascular space.  IgM are the predominant class of antibodies in primary response.  Natural antibodies are IgM in nature. Thus, a person having blood group A antigen will have anti-B antibodies in his circulation (isohemagglutinins).  These are produced without any known antigenic stimulation, and hence called natural antibodies. These IgM antibodies cannot cross placenta, and therefore the fetus, even though it carries an incompatible antigen, is protected from natural antibodies of the mother.

IMMUNOGLOBULIN A (IgA):  IgA usually are dimers (total 4 heavy chains and 4 light chains). The J chain connects the dimers.  They are the secretory antibodies seen in seromucous secretions of gastrointestinal tract, nasopharyngeal tract, urogenital tract, tears, saliva, sweat, among others.  The dimers are stabilized against proteolytic enzymes by the secretory piece. The secretory piece is produced in liver, reaches to the intestinal mucosal cells, where it combines with IgA dimer to form the secretory IgA which is then released.

IMMUNOGLOBULIN E (IgE):  They are cytophilic antibodies. They mediate allergy, hypersensitivity and anaphylaxis.  They have the property to fix on mast cells and basophils. When certain antigens such as penicillin are injected a few times, IgE class antibodies are produced which anchor on mast cells. When the same chemical is injected next time, the antigens fix on such antibodies, causing mast cell degranulation, and release of histamine and slow reacting substance.  This leads to vasodilatation, hypotension and bronchiolar constriction. This is the basis of penicillin anaphylaxis, hay fever caused by fungus, asthma by pollen and urticaria by absorbed food elements.  The peak of this reaction will be at about 30 minutes; hence called immediate type hypersensitivity.  IgE level in serum is markedly increased in helminthic infections.

 

RELATED;

1.  HAY FEVER  

2. PASSIVE IMMUNISATION  

3. HAEMOLYTIC DISEASE OF THE NEW BORN

4.  SPECIFIC IMMUNITY

REFERENCES