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Infant age

Months

Figure 16.7 Immunoglobulin G Levels in the Fetus and Infant During gestation, maternal IgG is transported across the placenta to the fetus. Colostrum also contains IgG. Normally, the fetus does not make appreciable amounts of immunoglobulin; it depends on the maternal antibodies that are transferred passively. After birth, the infant begins to produce immunoglobulins to replace the maternal antibodies, which gradually disappear over a period of about 6 months. Usually by 3 to 6 months, most of the antibodies present are those produced by the infant.

than being free in the circulation. The bound IgE molecules allow these cells to detect and respond to antigens. For example, when antigen binds to two adjacent IgE molecules carried by a mast cell, the cell releases a mixture of potent chemicals including histamine, cytokines, and various compounds that contribute to the inflammatory response. Evidence suggests that these responses are important in the elimination of parasites, particularly helminths. ■ helminths, p. 317

Unfortunately for allergy sufferers, basophils and mast cells also release their chemicals when IgE binds to normally harmless materials such as dusts and pollens, leading to immediate reactions such as coughing, sneezing, and muscular contractions. In some cases these allergic, or hypersensitivy, reactions can be life-threatening. ■ hypersensitivity reactions, p. 441

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