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Classes of Immunoglobulins
Immunoglobulins (antibodies) are grouped into five classes (IgG, IgM, IgA, IgD and IgE) based on their unique antigenic properties, which reside in their heavy chains (in the constant region).

The heavy chains of an Ig class are designated by the small Greek Letter corresponding to the class of Ig, i.e., 'Y heavy chain in IgG, /.l in IgM, ex in IgA, 0 in IgD and E in IgE. An individual has antibodies of all the five classes of Ig.

The immunoglobulins are further divided into subclasses based on antigenic differences within each of the five classes. In humans, IgG has four subclasses (IgGI, IgG2, IgG3, IgG4), Ig A has to subclasses (IgAl, IgA2), while IgM, IgD and IgE have no subclasses.
IgG is a monomer, makes up Ca 75% of the total human Ig, and is the only class of Ig, which can cross the placenta and protect the newborn. IgM constitutes 10% of human Ig, is a pentamer of the monomeric units and has a fourth CH domain (CH4). IgA is of two types: serum IgA (present in serum at 20% of total Ig) and secretary IgA (predominant Ig in saliva, colostrum, milk, genitourinary secretions). IgD is very low (0.03 mg/ ml) in the serum.

Both IgD and IgM are present on the surface membranes of mature but virgin B lymphocytes. The IgD and IgM present on the surface of a single lymphocyte cell have the same antigen binding specificity. IgE is involved in hypersensitivity and allergy, and is only 0.00005 mg/ml in the serum.
The Fc portion of IgE binds strongly to a receptor on mast cells, which is involved in the allergic reaction.

 
 

 
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