Passive (Artificially Acquired) Immunity

PASSIVE (ARTIFICIALLY ACQUIRED) IMMUNITY

Humoral antibodies of the IgG class are able to cross the placenta from mother to fetus. These antibodies will provide passive protection of the newborn against those diseases which involve humoral immunity and to which the mother is immune. In this manner, most newborn infants in the UK will have passive protection against tetanus, but not against tuberculosis. Protection against the latter relies to a large extent on cell-mediated immunity. Secreted (IgA) antibodies are also passed to the gut of newborn, together with the first deliveries of breast milk (colostrum). Such antibodies provide some passive protection against infections of the gastrointestinal tract. Maternally acquired antibodies will react with antigens associated with an infection but also with antigens introduced to the body as part of an immunization programme. Premature immunization, i.e. before degradation of the maternal antibodies, may reduce the potency of an administered vaccine. This aspect of the timing of a course of vaccinations is discussed later.

Administration of preformed antibodies taken from animals, pooled human serum, or human cell lines is often used to treat existing infections (e.g. tetanus, diphtheria) or condition (e.g. venomous snake bite). Pooled serum may also be administered prophylactically, within a slow-release vehicle, for individuals travelling to parts of the world where diseases such as hepatitis A are endemic. Such administrations confer no long-term immunity and may interfere with concurrent vaccination procedures.