Naturally acquired active immunity

Exposure to live pathogens

Symptoms of a disease and stimulation of an immune response

Artificially acquired active immunity

Exposure to a vaccine containing weakened or dead pathogens or their components

Stimulation of an immune response without the severe symptoms of a disease

Artificially acquired passive immunity

Injection of antibodies or antitoxin

Immunity for a short time without stimulating an immune response

Naturally acquired passive immunity

Antibodies passed to fetus from pregnant woman with active immunity

Short-term immunity for infant without stimulating an immune response

During pregnancy, certain antibodies (IgG) pass from the maternal blood into the fetal bloodstream. Receptor-mediated endocytosis (see chapter 3, p. 91) utilizing receptor sites on cells of the fetal yolk sac accomplishes the transfer (see chapter 23, p. 950). These receptor sites bind to a region common to the structure of IgG molecules. After entering the fetal cells, the antibodies are secreted into the fetal blood. As a result, the fetus acquires limited immunity against the pathogens for which the pregnant woman has developed active immunities. Thus, the fetus has naturally acquired passive immunity, which may persist for six months to a year after birth. Table 16.8 summarizes the types of immunity.

O Distinguish between a primary and a secondary immune response.

Explain the difference between active and passive immunities.

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