An antibody or immunoglobulin (Ig) is glycoprotein produced by B cells, which is capable of combining specifically with the antigen, which induces it. Antibodies are divided into five major classes, IgM, IgG, IgA, IgD and IgE, based on their heavy chain constant region structure. An antibody has at least two identical antigen-bindingsites and it is the valence of an antibody.
IgG has two light chains either kappa or lambda and two heavy chain ofgtypeand consists of four subclasses IgG1, IgG2, IgG3 and IgG4. It is predominant class of immunoglobulin and account for approximately 80% in human serum. IgG produced particularly during the secondary immune response. IgG stimulates phagocytic cells, activates the complement system, binds neutrophils, and can neutralize toxins. Most importantly, it is the only antibody that can cross the placenta and confer immunity on the foetus.
IgA has two light chains either kappa or lambda and two heavy chain ofatypeand consist of two subclasses IgA1 and IgA2, constitutes only 13% of the antibody in human serum, but predominant class of antibody in extravascular secretions. The IgA present in secretions (tears, saliva, nasal secretions and mammary gland secretions) is secretory IgA. It is found to produce immunity against tapeworms and present in the colostrums protects the baby from intestinal pathogens.
IgM has two light chains either kappa or lambda and two heavy chain ofmtypeconstitutes 8% of the antibody in human serum, it is the largest of the immunoglobulins often referred as the macroglobulin because it has more than five binding sites for antigen. It is the first antibody to appear in the primary immune response therefore an useful indicator of recent infection. Most of the natural antibodies like ABO blood grouping (anti-A anti-B) are of the IgM class and important in the initial activation of B-cells, macrophages, and activate the complement system.
IgD has two light chains either kappa or lambda and two heavy chain ofdtypeconstitute less than 1% of the antibody in human serum. Plays a role in activating and suppressing lymphocyte activity and found large quantities in the cell walls of many B-cells. IgD has a single binding site.
IgE is a reaginic antibody, has two light chains either kappa or lambda andtwo heavy chain of e type constitute less than 0.003% of the antibody in human serum. Mediator in allergic responses. Most importantly activates histamine secreting cells. Also appears to play a role in parasitic infection and mediates type one hypersensitivity.