Immunoglobulins
Immunoglobulin is a general term for a protein that can bind to an antigen; it includes both antibodies (circulating immunoglobulins) and those found in the cell membranes of B cells.
Immunoglobulins fall into five major classes based on their chemical and functional characteristics (Table 16-1). Immunoglobulin (Ig) G is the predominant circulating immunoglobulin. IgG has a variety of functions, including (1) binding free circulating antigens to reduce their potential for harm (neutralization) and promote their removal by phagocytes, (2) binding antigens associated with bacterial cell walls and promoting their phagocytosis, (3) binding antigens and activating complement to promote inflammation and destruction of harmful microbes, and (4) acting as an agglutinin to clump particulates so that they can be more effectively phagocytized. IgG does not actually destroy any harmful substance or agent; it promotes destruction or removal indirectly.IgE is the class of immunoglobulins associated with most allergic responses. When an allergen (an agent capable of inducing an allergic response) is first encountered, specific IgE is produced and incorporates into the cell membrane of mast cells. When the same allergen is again encountered, the allergen binds to the specific IgE on the mast cells, and this stimulates the mast cells to release their secretory granules containing histamine and produce leukotrienes. These agents bring about many of the cell and tissue responses characteristic of allergies.
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