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The Anatomy of Antibody-Mediated Etiologies of the Neuropathies

© Les Abrams

IgA

IgA is found, in low levels, in the monomeric form (concentration is usually around 3.5 mg/ml) but IgA is most common and most active at mucosal surfaces where it appears as a dimeric protein. The two IgA are held together by a J chain (not the same as the J region of the antibody gene). To pass through epithelial surfaces secretory component is transiently attached to dimeric IgA. The dimeric IgA provides the primary defense at mucosal surfaces such as bronchioles, nasal mucosa, prostate, vagina, and intestine. IgA is also abundant in saliva, tears and breast milk, especially colostrum.

IgD

IgD is found on the surface of most B-lymphocytes just like sIgM. Until now, the function of this antibody is unknown but it has been suggested that it acts as an antigen receptor and that it is needed for B cell activation. A very small amount of IgD is secreted, and its functions as a secreted antibody are unclear.

IgE

Like IgA, IgE is particularly effective at mucosal surfaces. It is also active in the blood and in the tissues. The serum concentration of this antibody is normally very low as most IgE is tightly bound to its receptors on mast cells and basophils. The production of IgE is controlled by cytokines and this class is responsible for Type I hypersentitivity reactions (allergic and anaphylactic). IgE is found to increase greatly in response to parasitic infection. It is also involved in inflammatory reponses through its role in mast cell degranulation.

IgG

IgG is the most abundant form (class) of antibody in the blood (serum concentration is 13 mg/ml). There are four subclasses of IgG, which are all monomeric and they usually have a very high affinity for antigen. Unlike IgM, IgG is able to leave the blood stream and enter tissues.

IgG is also the only class of antibody to pass the placental barrier. Therefore IgG provides the only antibody protection for newborns until their own immune system is able to contribute to antibody production.

The subclasses of antibody IgG produced is dependant on the cytokines present (especially IL-4 and IL-2) and each class has its own special activity. In general, IgG is very good at activating complement, and is the best antibody for opsonization using Fc receptors on phagocytes. This is because phagocytes have a large number of Fc gamma receptors on their surface. IgG also plays an important role in neutralizing toxins (from bacterial infection for example) in the blood and tissues.