What are the differences between IgG, IgM, IgE and IgD?

IgG, IgM, IgE, and IgD are four of five classes of antibodies found in placental mammals. Each of these antibodies differ significantly in their structure and function. 

IgG

IgG is the most predominant type of antibody circulating in the blood, with 75% serum concentration. IgG antibodies have the longest serum half-life and the highest opsonization and neutralization activities. IgG antibodies are responsible for facilitating phagocytic destruction of pathogens.   Their main function is to provide the main humoral immune response against pathogenic antigens that invade the body. They also play a vital role in the secondary immune response and in antibody-dependent cell-mediated cytotoxicity. IgG has four subclasses IgG1, IgG2, IgG3, and IgG4. It is the only form of antibody that is capable of crossing the placenta membrane, thereby providing passive immunity to the fetus. 

IgM

IgM is the first form of antibody produced and released rapidly into the bloodstream after an invasion of pathogenic antigens. IgM is the large form of antibody and the only one that occurs as a pentamer. Because of its pentameric structure, the IgM molecule has 10 antigen-binding sites, which gives it a higher overall binding strength or avidity as compared to IgG even though it has a comparatively lower affinity towards antigens. IgM antibodies have a 10% serum concentration. 

IgE

IgE antibodies are produced by the immune system in response to an allergy. They bind with allergens to initiate the release of histamine from basophils and mast cells, resulting in an allergic reaction. IgE is the form of antibody with the lowest serum concentration at less than 0.01%. It also has the shortest half-life in the serum. Despite their low concentrations and short half-life, it is a very potent antibody associated with hypersensitivity and allergies.  

IgD

IgD are antibodies that are expressed in the plasma membranes of immature B lymphocytes. They function as antigen receptors on activated B cells. IgD has the second-lowest serum concentration at less than 0.5% and a relatively short half-life in the serum. It occurs as a monomer and has a hinge region that undergoes proteolysis.