• Passive immunity

Antibodies produced by active immunity in one individual can be transferred in serum to a non-immune recipient. The antibody recipient is then said to have passive immunity. This type of immunity is characterized as follows:

1. Although these antibodies provide immediate protection, they are eventually depleted and are not replaced by the body.

2. Passive immunization, therefore, provides only temporary, short-term protection, usually lasting no more than a few weeks. This is commonly used for immunotherapy. Cell-mediated immunity can be passively transferred by immune lymphocytes derived from a sensitized donor.

Natural passive immunity helps to protect newborns from infectious disease. The fetus, which is incapable of producing antibodies, becomes passively immunized by maternal antibodies that cross the placenta from the mother to the fetus. Maternal antibodies persist for many weeks following birth and help protect the baby until he or she can actively produce antibodies. (Immunologic competence begins during the third to sixth month after birth and may not be adequate for 2 or 3 years.) Additional protection is provided by maternal antibodies that are passively transferred in breast milk. Breast milk also contains 25 percent of the mother’s daily production of monocytes. Such protection cannot be acquired from a commercial infant formula or cow's milk.

Artificial passive immunity is a valuable therapeutic tool when immediate protection is required, with no time to wait for active immunity to develop. Antibodies or immune T lymphocytes produced by hyperimmunized individual (human or animal) are injected into a recipient to provide temporary protection.

Antibodies of known specificity for passive immunization are administered as hyperimmune serum (antiserum), or specific immunoglobulins obtained from persons who have been actively immunized with an appropriate vaccine (see Table 15-6 and Table 15-7).

The main features of preparations for artificial passive immunity are as follows:

• Antiserum of human origin is called homologous, and antibodies that are taken from hyperimmunized animal are named heterologous.

• The antiserum (or immunoglobulin) may be specific (immune) or non-specific (normal). Hyperimmune human antisera are available for use against rabies, measles, pertussis, and tetanus. Pooled normal human immunoglobulin contains non-specific antibody to several pathogens and it is prepared from batches of plasma from 1000-6000 healthy donors after screening for hepatitis B, C and HIV.

• Immune antisera may contain antimicrobial or antitoxic antibodies, so such types of antisera may develop antimicrobial or antitoxic passive immunity.

• Protective antibodies can be prepared against several exotoxins that are lethal unless quickly neutralized in the body. Antiserum is available for use against botulism, tetanus, and the poisonous bites of spiders and snakes. If the antiserum contains antibodies that neutralize toxin, it is called antitoxin.

• Antitoxic immunoglobulins are produced by actively immunizing a horse with tetanus toxoid and concentrating the gamma-globulin fraction from blood collected from the animal several weeks later. Patients in danger of developing tetanus are injected with this antitoxin preparation, which rapidly neutralizes the potentially lethal exotoxin; these persons could otherwise die of the disease before they produced their own antibodies.

NOTE: This is why an injured patient will sometimes receive two tetanus shots, one a toxoid to boost active immunity for subsequent years of protection, the other an antitoxin preparation to provide immediate protection against the toxin. The two shots are given in different arms so they don't neutralize each other.

• monoclonal antibodies. Pure monospecific antibodies can be produced in cell culture in which all the cells in the population are descendants of a single parental antibody-producing cell. All the cells in this clone produce identical products. A culture grown from one isolated В cell produces immunoglobulin specific for a single antigenic determinant. Immunoglobulin manufactured by cloned cell cultures is called monoclonal antibody. A myeloma cell and an antigen-specific В lymphocyte can be fused to form a single cell, a hybridoma, which is committed to the production of the single antibody type specified by the В cell. Monoclonal antibodies are currently used in immunotherapy, diagnostic kits, and as a "guided missile" that delivers cytotoxic chemicals directly to tumor cells that bear the target antigen.