Immunology

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1. The five classes of immunoglobulins and their functions: -

(1) Immunoglobulin M (IgM)

The IgM molecule is found predominantly in the intra vascular compartment and on the surface of B lymphocytes and does not normally cross the placenta. IgM antibody predominates in early, primary immune responses; carbohydrate antigens such as blood group antigens stimulate IgM.

(2) Immunoglobulin A (IgA)

IgA is present in blood and in relatively high concentrations in saliva, colostrum, tears, and secretions of the bronchi and gastrointestinal tract. Secretory IgA plays an important role in host defense against viral and bacterial infections by blockingtransport of microbes across mucosa.

(3) Immunoglobulin G (IgG)

IgG comprises about 70% of total serum immunoglobulins and is distributed in the extracellular fluid; it is the only immunoglobulin that normally crosses the placenta. Antigen-bound IgG fixes complement via the Fc region of the constant chain. Immune effector cells express Fc receptors and complement receptors that facilitate phagocytosis and cytolysis. Immune complex activation of the classic complement pathway also generates soluble factors that chemoattract neutrophils, increase vascular permeability, and amplify the inflammatory response.

(4) Immunoglobulin E (IgE)

IgE is present in serum in very low concentrations as a single immunoglobulin unit with heavy chains. Fifty percent of patients with allergic diseases have increased serum IgE levels. The specific interaction between antigen and mast cell-bound IgE results in the release of histamine, leukotrienes, proteases, chemotactic factors, and cytokines. These mediators can produce bronchospasm, vasodilation, increased vascular permeability, smooth muscle contraction, and chemoattraction of ...