IgM is the first class of antibody to appear in response to a foreign antigen. Since IgM is a pentamer, it is polyreactive and an avid binder. IgM is also a powerful activator of the complement system (Kimball, 1990). The presence of IgM in the absence of antigenic stimulation, natural IgM, is thought to play an important role in innate immunity and the immediate defense against bacterial infection (Boes, Prodeus, Schmidt, Carroll, & Chen, 1998).
IgG is the most abundant immunoglobulin and is equally distributed in blood and tissue (Junqueira, Luiz C. and Jose Carneiro, 2003). IgG is primarily involved in the second class of antibodies to appear in response to a foreign antigen. The presence of IgG usually signifies a mature antibody response (Meulenbroek, A.J and Zeijlemaker, W.P,1996). IgG can bind to many pathogens and also plays an important role in antibody-dependent cell-mediated cytotoxicity. For example, congenital and acquired immunodeficiency results in significantly reduced levels of IgG, while hepatitis, cirrhosis, and most autoimmune diseases increase the IgG concentrations.
IgE is the least abundant form of the total immunoglobulins in serum. Although the blood levels of IgE are typically only 0.05% of IgG concentrations, IgE is capable of triggering the most powerful immune response. IgE plays an important role in atopic allergy diseases, such as asthma, dermatitis, and hay fever. IgE is typically associated with Type 1Hypersensitivity, which generally causes itching and inflammation in skin reactions and congestion in lung and bronchial reactions. IgE has also been implicated in immune responses to parasitic worms and certain protozoan parasites. Decreased levels of IgE can be found in cases of autoimmune disease, cancer, hepatitis, and malaria.
IgA is the major immunoglobulin found in secretory fluids such as saliva, tears, and colostrum. Secretory IgA is also the most abundant immunoglobulin in the respiratory and intestinal mucosa where it serves as a first line of defense against bacterial and viral invasion from the external environment.