Isotypes and Subclasses

In mammals, antibodies are classified into five main classes or isotypes– IgA, IgD, IgE, IgG, and IgM. The classification is determined by the constant regions of the heavy chains.


IgA: Immunoglobulin A (IgA) is the most abundant type of antibody in the body, comprising most of the immunoglobulin in secretions and a significant amount of circulating immunoglobulin. In secretions, it serves to protect the mucosal tissues from microbial invasion and maintain immune homeostasis with the microbiota.


IgD: Immunoglobulin D (IgD) is primarily found on the surface of B lymphocytes, where it functions as a receptor for antigens. It may participate in B cell maturation, maintenance, activation, and silencing. Although the exact function is still unclear, IgD may be involved in humoral immune responses by regulating B cell selection and homeostasis.


IgE: Immunoglobulin E (IgE) is most commonly associated with allergic disease and is thought to mediate an exaggerated and/or maladaptive immune response to antigens. 


IgG: Immunoglobulin G (IgG) neutralizes pathogens such as viruses and bacteria by binding to key pathogen surface proteins and preventing interaction of the pathogen with host cells. In doing so, the antibody neutralizes the ability of the pathogen to enter host cells and replicate.


IgM: Immunoglobulin M (IgM) not only serves as the first line of host defense against infections but also plays an important role in immune regulation and immunological tolerance. For many years, IgM has been thought to function by binding to antigens and activating the complement system.


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Kelly, Brian T., and Mitchell H. Grayson. "IgE, what is it good for?." Annals of allergy, asthma & immunology: official publication of the American College of Allergy, Asthma, & Immunology 116.3 (2016): 183.

Temming, A. Robin, et al. "Cross-reactivity of mouse IgG subclasses to human Fc gamma receptors: Antibody deglycosylation only eliminates IgG2b binding." Molecular Immunology 127 (2020): 79-86.

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