Passive immunity

Administration of serum to uninfected individual

Serum (antibodies) from immune individual

Administration of serum to uninfected individual

Infection

Recovery (immunity)

Recovery (immunity)

FIGURE 1-3 Active and passive immunity. Active immunity is conferred by a host response to a microbe or microbial antigen, whereas passive immunity is conferred by adoptive transfer of antibodies or T lymphocytes specific for the microbe. Both forms of immunity provide resistance to infection and are specific for microbial antigens, but only active immune responses generate immunologic memory. Cell transfers can be done only between genetically identical donor and recipient (e.g., inbred mice) to avoid rejection of the transferred cells.

recipient of such a transfer becomes immune to the particular antigen without ever having been exposed to or having responded to that antigen. Therefore, this form of immunity is called passive immunity. Passive immunization is a useful method for conferring resistance rapidly, without having to wait for an active immune response to develop. A physiologically important example of passive immunity is the transfer of maternal antibodies to the fetus, which enables newborns to combat infections before they develop the ability to produce antibodies themselves. Passive immunization against toxins by the administration of antibodies from immunized animals is a lifesaving treatment for potentially lethal infections, such as tetanus, and snake bites. The technique of adoptive transfer has also made it possible to define the various cells and molecules that are responsible for mediating specific immunity. In fact, humoral immunity was originally defined as the type of immunity that could be transferred to unimmunized, or naive, individuals by antibody-containing cell-free portions of the blood (i.e., plasma or serum) obtained from previously immunized individuals. Similarly, cell-mediated immunity was defined as the form of immunity that can be transferred to naive animals with cells (T lymphocytes) from immunized animals but not with plasma or serum.

The first experimental demonstration of humoral immunity was provided by Emil von Behring and Shi-basaburo Kitasato in 1890. They showed that if serum from animals that had recovered from diphtheria infection was transferred to naive animals, the recipients became specifically resistant to diphtheria infection. The active components of the serum were called antitoxins because they neutralized the pathologic effects of the diphtheria toxin. This result led to the treatment of otherwise lethal diphtheria infection by the administration of antitoxin, an achievement that was recognized by the award of the first Nobel Prize in Physiology or Medicine to von Behring. In the early 1900s, Paul Ehrlich postulated that immune cells use receptors, which he called side chains, to recognize microbial toxins and subsequently secrete these receptors to combat microbes. He also coined the term antibodies (antikorper in German) for the serum proteins that bound toxins, and substances that stimulated the production of antibodies were called antigens. The modern definition of antigens includes substances that bind to specific lymphocyte receptors, whether or not they stimulate immune responses. According to strict definitions, substances that stimulate immune responses are called immunogens. The properties of antibodies and antigens are described in Chapter 5. Ehrlich's concepts are a remarkably prescient model for the function of B cells in humoral immunity. This early emphasis on antibodies led to the general acceptance of the humoral theory of immunity, according to which host defense against infections is mediated by substances present in body fluids (once called humors).

The cellular theory of immunity, which stated that host cells are the principal mediators of immunity, was championed initially by Elie Metchnikoff. His demonstration of phagocytes surrounding a thorn stuck into a translucent starfish larva, published in 1883, was perhaps the first experimental evidence that cells respond to foreign invaders. Ehrlich and Metchnikoff shared the Nobel Prize in 1908, in recognition of their contributions to establishing these fundamental principles of immunity. Sir Almroth Wright's observation in the early 1900s that factors in immune serum enhanced the phagocytosis of bacteria by coating the bacteria, a process known as opsonization, lent support to the belief that antibodies prepared microbes for ingestion by phagocytes. These early "cellularists" were unable to prove that specific immunity to microbes could be mediated by cells. The cellular theory of immunity became firmly established in the 1950s, when it was shown that resistance to an intracellular bacterium, Listeria monocytogenes, could be adoptively transferred with cells but not with serum. We now know that the specificity of cell-mediated immunity is due to lymphocytes, which often function in concert with other cells, such as phagocytes, to eliminate microbes.

In the clinical setting, immunity to a previously encountered microbe is measured indirectly, either by assaying for the presence of products of immune responses (such as serum antibodies specific for microbial antigens) or by administering substances purified from the microbe and measuring reactions to these substances. A reaction to a microbial antigen is detectable only in individuals who have previously encountered the antigen; these individuals are said to be "sensitized" to the antigen, and the reaction is an indication of "sensitivity." Although the reaction to the purified antigen has no protective function, it implies that the sensitized individual is capable of mounting a protective immune response to the microbe.

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