Immunoglobulin class switching

Immunoglobulin class switching, also known as isotype switching, is a biological mechanism that changes a B cell's production of immunoglobulin (antibody) from one type to another, such as from IgM to IgG. This process is crucial for the adaptive immune system to produce antibodies that are more effective in neutralizing pathogens.

Mechanism

Class switching occurs in the germinal centers of lymphoid tissues during the immune response. It involves a recombination event at the DNA level, specifically within the immunoglobulin heavy chain locus. The process is initiated by activation-induced cytidine deaminase (AID), which introduces single-strand breaks in the DNA at the switch regions located upstream of each constant region gene segment.

Types of Immunoglobulins

The main classes of immunoglobulins that can be produced through class switching include:

• IgM: The first antibody produced in response to an infection.
• IgG: The most abundant antibody in the blood and extracellular fluid, providing long-term protection.
• IgA: Found in mucosal areas, such as the gut, respiratory tract, and urogenital tract, as well as in secretions like saliva and breast milk.
• IgE: Involved in allergic reactions and protection against parasitic infections.
• IgD: Functions mainly as a receptor on B cells that have not been exposed to antigens.

Regulation

Class switching is regulated by cytokines produced by T helper cells. Different cytokines promote switching to different isotypes. For example:

• Interleukin-4 (IL-4) promotes switching to IgE and IgG1.
• Interferon-gamma (IFN-γ) promotes switching to IgG2a.
• Transforming growth factor-beta (TGF-β) promotes switching to IgA.

Clinical Significance

Defects in class switching can lead to immunodeficiency disorders, such as Hyper IgM syndrome, where B cells are unable to switch from producing IgM to other isotypes. This results in increased susceptibility to infections.

See Also

• Adaptive immune system
• B cell receptor
• Somatic hypermutation
• Germinal center