Passive Immunity

The term passive immunity refers to the immune protection that can be produced by the transfer of antibodies to a recipient from a human or animal donor. The donor has been actively immunized, as explained by the clonal selection theory. The person who receives these ready-made antibodies is thus passively immunized to the same antigens. Passive immunity also occurs naturally in the transfer of immunity from mother to fetus during pregnancy and from mother to baby during nursing.

The ability to mount a specific immune response—called immunological competence—does not develop until about a month after birth. The fetus, therefore, cannot immunologically reject its mother. The immune system of the mother is fully competent but does not usually respond to fetal antigens for reasons that are not completely understood. Some IgG antibodies from the mother do cross the placenta and enter the fetal circulation, however, and these serve to confer passive immunity to the fetus.

The fetus and the newborn baby are thus immune to the same antigens as the mother. However, since the baby did not itself produce the lymphocyte clones needed to form these antibodies, such passive immunity gradually disappears. If the baby is breastfed it can receive additional antibodies of the IgA subclass in its mother's milk and colostrum (the secretion an infant feeds on for the first 2 or 3 days until the onset of true lactation). This provides additional passive immunity until the baby can produce its own antibodies through active immunity (see chapter 20, fig. 20.56).

Passive immunizations are used clinically to protect people who have been exposed to extremely virulent infections or toxins, such as tetanus, hepatitis, rabies, and snake venom. In these cases, the affected person is injected with antiserum (serum containing antibodies), also called antitoxin, from an animal that has been previously exposed to the pathogen. The animal develops the lymphocyte clones and active immunity, and thus has a high concentration of antibodies in its blood. Since the person who is injected with these antibodies does not develop active immunity, he or she must again be injected with antitoxin upon subsequent exposures.

Active and passive immunity are compared in table 15.9.

1 Fox: Human Physiology, Eighth Edition

1 15. The Immune System 1 Text 1

© The McGraw-Hill Companies, 2003

The Immune System

467

Table 15.9 Comparison of Active and Passive Immunity

Characteristic Active Immunity

Passive Immunity

Injection of person with Source of antibodies Method

Antigens

The person inoculated Injection with killed or attenuated pathogens or their toxins

Antibodies

Natural—the mother; artificial—injection with antibodies Natural—transfer of antibodies across the placenta; artificial— injection with antibodies

Time to develop resistance

5 to 14 days

Immediately after injection

Duration of resistance When used

Long (perhaps years) Before exposure to pathogen

Short (days to weeks)

Before or after exposure to pathogen

Myeloma cell culture

Myeloma cell culture

Myeloma cells

Fusion

Myeloma cells

Fusion

Immunization

Immunization on

B lymphocytes from spleen

Clone antibody-producing (positive) hybrids

Selection of y hybrid cells /

Monoclonal antibody on

Hybridoma cell

Assay for antibody

Selection of y hybrid cells /

Assay for antibody

Immune Serum Artificial Passive Immunity

Monoclonal antibody

Monoclonal antibody

Freeze hybridoma for future use

Monoclonal antibody

Freeze hybridoma for future use

■ Figure 15.24 The production of monoclonal antibodies. These are antibodies produced by the progeny of a single B lymphocyte, so that all of the antibodies are directed against a specific antigen.

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Responses

  • carolyn
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    7 years ago
  • Gary
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    6 years ago
  • elfstan
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    6 years ago
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    5 years ago
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    10 months ago

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