IgD is a monomer bound within the membrane of B cells. It serves as an antigen-binding receptor. IgD is not secreted by B cells, instead it comes from the degradation of old B cells and the release of IgD molecules from their cytoplasmic membranes.

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The constant region of an antibody molecule determines its class, or isotype. The five classes of antibodies are:
- IgG
- IgM
- IgA
- IgD
- IgE

Classes of Antibodies 

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IgG is a monomer and the most abundant antibody in body. IgG penetrates into tissue spaces It is the only antibody class with the ability to cross the placenta, providing passive immunity to the developing fetus during pregnancy.

Antibody IgG

IgM is initially produced  as a monomer bound within the membrane.It serves as an antigen-binding receptor on B cells. When secreted, IgM assembles into a pentamer with five monomers of IgM bound together by a protein structure called the J chain. IgM is the first antibody produced and secreted by B cells during the primary and secondary immune responses, making pathogen-specific IgM a valuable diagnostic marker during active or recent infections.

Antibody IgM

IgA is the most common antibody class found in the mucus secretions that protect the mucous membranes. IgA can also be found in other secretions such as breast milk, passing immunity to newborns. Secretory IgA is assembled into a dimeric form with two monomers joined by a protein structure called the secretory component. One of the important functions of secretory IgA is to trap pathogens in mucus so that they can later be removed from the body.

Antibody IgA

Antibody IgD

IgE is the least abundant antibody in the body. It is secreted as a monomer, and its role in adaptive immunity is related to anti-parasitic defenses. The Fc region binds to basophils and mast cells. The Fab region of the bound IgE then interacts with specific antigen epitopes, causing the cells to release potent pro-inflammatory mediators.

Antibody IgE

Hypersensitivity is an exaggerated response that sometimes occurs when an antibody interacts with an antigen.

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