There is abundant evidence for the efficacy of antibody in preventing infection. For example, artificial passive immunization (injection of antibodies) temporarily protects against hepatitis A or B, rabies, measles, varicella, and several other viral infections (see Chapter 13). Furthermore, natural passive immunization protects the newborn for the first few months of life against most of the infections that the mother has experienced. In humans this occurs in two ways: (1) maternal antibodies of the IgG class cross the placenta and protect the fetus and the newborn infant during pregnancy and for several months after birth; (2) antibodies of the fgA class are secreted in the mother's milk at a concentration of 1.5 grams per liter (and considerably higher in the early colostrum), conferring protection against enteric infections as long as breast-feeding continues. If the infant encounters viruses when maternal immunity is waning, the virus replicates to only a limited extent, causing no significant disease but stimulating an immune response; thus the infant acquires active immunity while partially protected by maternal immunity. Maternally derived antibody also interferes with active immunization of the newborn and must therefore be taken into account when designing vaccination schedules (see Chapter 13).
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