IgE is present in the lowest concentration in the serum. It's basic function is in allergic reactions. When a allergic antigen enters the body, IgE bind to mast cells which leads to the degranulation of these cells. And they release substances like histamine and heparin which cause the symptoms of allergy like vomiting, sneezing, diarrhoea , hives, asthma, hay fever and anaphylactic death. This immunoglobulin is also linked to allergic hypersensitivity. IgE also plays an additional role in anti parasitic reactions. It is not passed through the placenta to the child. 

State University of New York at Buffalo

Immunoglobulins are proteins that act as antibodies to recognise antigens, attach to them and make it easier for phagocytic cells to kill them. There are five classes of immunoglobulins that are distinguished by the type of heavy chains found on the molecule:
1. IgG
2.IgM
3.IgA
4.IgE
5.IgD

Different classes of immunoglobulins

IgG is found in the serum in the highest concentration as compared to other immunoglobulins. IgG helps fight infections in serum by opsonisation, complement activation, neutralising toxins and viruses and causes agglutination /precipitation by cross linking. IgG has a long life and is passed from the mother to the fetus through the umbilical cord. If there is presence of IgG in the blood we can conclude that there is an ongoing infection. IgG is further divided into 4 subclasses: IgG1, IgG2, IgG3, IgG4.

Function of IgG

IgM exists as a pentamer composed of 5 of the 2 heavy chains and 2 light chain units. It is the largest immunoglobulin, and is called a macroglobulin due to its large size. It is the first immunoglobulin to be produced in a new born. Elevated levels of IgM at birth indicate an infection. It is not passed onto the newborn through the placenta. It has a short life. IgM also plays an important role in B cell maturation. 

Functions of IgM

IgA is the second highest immunoglobulin in serum concentration. It exists as a monomer in the serum where it has a minimal function. The primary function of IgA is in the secretions, where it exists as a dimer, it prevents pathogens from entering the body. It also prevents the adhesion of bacteria, virus and toxins. IgA is not passed through the placenta from the mother to the child, however it is present in the colostrum (i.e. the initial milk produced by the mother). There are two sub classes of IgA: IgA1 and IgA2. 

Functions of IgA

Functions of IgE

IgD is found on the surface of mature B cells. It plays a role in helping B cells respond to signals from T cells and also helps B cells in IgA, IgE and IgG synthesis. These antibodies are involved in antigen-specific B cell activation and are maturity markers for B cells. IgD binds to basophils and mast cells to cause activation of the cells. They are not passed through the placenta. 

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