Five classes of immunoglobulins
Humans have ?ve various classes of antibody molecule that differ both in structure and in function. These are known as immunoglobulin D, immunoglobulin E, immunoglobulin A, immunoglobulin G, and immunoglobulin M and each has its own type of heavy chain; α, δ, ε, γ and µ respectively. Therefore IgA molecules have two identical heavy chains, immunoglobulin D molecules have two identical heavy chains, etc. The human immunoglobulin G class of antibodies is further separated into four immunoglobulin G Sub groups; IgG1, IgG2, IgG3 and IgG4, which have γ1, γ2, γ3 or γ4 heavy chains respectively.
The several heavy chains confer many properties and functions on every of the immunoglobulin classes:
- IgM has heavy chains and exists as a pentamer in combination with another polypeptide known as the J chain, that is responsible for initiating the polymerization to form the pentameric structure. With its huge number of antigen-binding sites, every immunoglobulin M molecule binds very tightly to any pathogen which has multiple copies of the similar antigen on its surface. The binding persuade the Fc region to activate the complement pathway that eventually causes the death of the pathogen. Immunoglobulin M also activates macrophages to phagocytose pathogens. Not unexpectedly given these functions immunoglobulin M is the ?rst antibody produced when an animal responds to a new antigen.
- Immunoglobulin G is the prime immunoglobulin in the bloodstream late in the primary immune response and particularly during the secondary immune response. Like immunoglobulin M, it can activate complement and trigger macrophages, but is the only antibody which can pass by the placenta and so provide immunological protection for the fetus. It is taken up from the gut of the newborn animal into the bloodstream and It is also secreted into the mother’s milk, therefore giving continuing protection after birth.
- Immunoglobulin A is the prime class of antibody in secretions like as saliva, tears and in secretions of the intestine and the lungs. It is the ?rst line of immunological defense against infection at these sites.
- Immunoglobulin E occurs in tissues where, having bound the antigen, it stimulates mast cells to release a range of factors. Several of these in turn activate white blood cells called eosinophils to kill several kinds of parasite. Moreover, the mast cells can also release biologically active amines, involving histamine, that cause dilation and increased permeability of blood vessels and lead to the symptoms seen in allergic reactions like as hay asthma and fever.
- Immunoglobulin D is known on the surface of mature B lymphocytes and in traces in several body ?uids, but its main function remains unclear.
There are two different forms of light chains also exist. The Antibody molecules in any of the antibody subclasses or classes can have either two light chains or two light chains. Unlike the variant heavy chains elaborate above no difference in biological function among and light chains are known.