Types of Antibodies Circulating in the Human Body
There are five primary types of antibodies (immunoglobulins) circulating in the human body: IgG, IgA, IgM, IgD, and IgE. 1
Distribution and Abundance
The five antibody classes are present in dramatically different proportions in normal serum:
- IgG comprises approximately 80% of total circulating immunoglobulins, making it the most abundant antibody class 1
- IgA accounts for approximately 15% of serum antibodies 1, 2
- IgM represents about 5-10% of circulating immunoglobulins 1, 2
- IgD constitutes only 0.2% of serum antibodies 1
- IgE is present in trace amounts, being the least abundant antibody class 1, 2
Functional Characteristics
IgG is the most clinically significant antibody due to its long half-life, ability to penetrate nearly all tissues, and capacity to trigger diverse effector functions against microbial pathogens 3. IgG deficiency frequently results in both increased susceptibility to infections and development of self-reactive antibodies, demonstrating its dual role in pathogen defense and maintaining self-tolerance 3.
IgA serves as the primary mucosal defense antibody, with unique structural features including two subclasses (IgA1 and IgA2) that differ in their hinge regions 4. IgA maintains immune homeostasis at mucosal barriers through immune exclusion of pathobionts while facilitating commensal colonization, with a large portion of gut microbiota being IgA-coated 4.
IgM, IgD, and IgE have specialized but less abundant roles in the immune response, with IgE being particularly important in allergic reactions despite its minimal serum concentration 1, 2.
Clinical Relevance
All five antibody classes are post-translationally modified by glycosylation, with terminal sialylation serving as an important regulator of antibody function across all classes 2. The quantitative measurement of the three major classes (IgG, IgA, and IgM) is routinely used clinically to diagnose conditions causing excess levels, deficiencies, or combinations of both 1.
Human immunoglobulin products are used for replacement therapy in primary and secondary immunodeficiency conditions, infection prevention and treatment, and as immunomodulatory agents for autoimmune and inflammatory disorders 5.