There are two types of adaptive immunity, called humoral immunity and cell-mediated immunity, which are mediated by different types of lymphocytes and function to eliminate different types of microbes (Figs. 1 and 2). Humoral immunity is mediated by molecules in the blood and mucosal secretions, called antibodies (also called immunoglobulins), which are produced by B lymphocytes. Antibodies recognize microbial antigens, neutralize the infectivity of the microbes, and target microbes for elimination by phagocytes and the complement system. Humoral immunity is the principal defense mechanism against microbes and their toxins located outside cells (e.g., in the lumens of the gastrointestinal and respiratory tracts and in the blood and extracellular tissue spaces) because secreted anti bodies can bind to these microbes and toxins, neutralize them, and assist in their elimination.

Fig1. Types of adaptive immunity. In humoral immunity, B lymphocytes secrete antibodies that pre vent infections and eliminate extracellular microbes. In cell-mediated immunity, helper T lymphocytes activate macrophages and neutrophils to kill phagocytosed microbes or cytotoxic T lymphocytes (CTLs) to directly destroy infected cells.

Fig2. Classes of lymphocytes. B lymphocytes recognize many different types of antigens and develop into antibody-secreting cells. Helper T lymphocytes recognize peptide fragments of protein antigens displayed on the surfaces of antigen-presenting cells and in response secrete cytokines, which stimulate different mechanisms of immunity and inflammation. Cytotoxic T lymphocytes recognize peptide fragments of protein antigens displayed on the surfaces of infected cells and kill these cells. Regulatory T cells suppress immune responses (e.g., to self antigens).

Cell-mediated immunity, also called cellular immunity, is mediated by T lymphocytes. Many microbes are ingested by but survive within phagocytes, and some microbes, notably viruses, infect and replicate in various host cells. In these locations the microbes are inaccessible to circulating antibodies. Defense against such infections is a function of cell-mediated immunity, which promotes the destruction of microbes inside phagocytes and the killing of infected cells to eliminate reservoirs of infection.

Different classes of lymphocytes may be distinguished by the expression of membrane proteins, many of which are designated by Cluster of Differentiation (CD) numbers. CD molecules are also involved in the functions of the lymphocytes.

Protective immunity against a microbe may be provided either by the host’s response to the microbe or by the transfer of antibodies from another individual (of the same or different species) that defend against the microbe (Fig. 3). The form of immunity that is induced by the host’s response to a foreign antigen is called active immunity. Individuals and lymphocytes that have not encountered a particular antigen are said to be naive, implying that they are immunologically inexperienced. Individuals who have responded to a microbial antigen and are protected from subsequent expo sures to that microbe are said to be immune.

Fig3. 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 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. Passive transfer of antibodies occurs during pregnancy (from mother to fetus), and injection of antibodies is used therapeutically to rapidly confer passive protective immunity against lethal toxins. Lymphocytes can be transferred only among genetically identical animals; in humans, lymphocytes from another individual would be recognized as foreign and rejected.

Immunity also can be conferred on an individual by transfer ring antibodies from an individual who had previously made an active immune response to an antigen into an individual who has not encountered the antigen (see Fig. 3). The recipient of such a transfer becomes immune to the particular antigen without ever having been exposed to or having responded to that antigen. This form of immunity is called passive immunity. A physiologically important example of passive immunity is the transfer of maternal antibodies through the placenta to the fetus, which enables newborns to combat infections for several months before they develop the ability to produce antibodies themselves. Passive immunization is also a medically useful method for conferring resistance rapidly without having to wait for an active immune response to develop. Passive immunization against potentially lethal toxins by the administration of antibodies from animals or people previously exposed to the toxins is a lifesaving treatment for rabies infection and snake bites. Patients with some genetic immunodeficiency diseases who cannot make their own antibodies are passively immunized by the transfer of pooled antibodies from healthy donors.

The first demonstration of humoral immunity was provided by Emil von Behring and Shibasaburo Kitasato in 1890, using a passive immunization strategy. They showed that if serum from animals that had been immunized with an attenuated form of diphtheria toxin was transferred to naive individuals, the recipients became 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 1890s 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 coined the term antibodies (antikörper in German) for the serum proteins that bound foreign substances, such as toxins, and the substances that generated antibodies were called antigens. The modern definition of antigens includes molecules 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, but antigen is often used interchangeably with immunogen. The properties of antibodies and antigens are described in Chapter 5. Ehrlich’s concepts were a remarkably prescient model for the specificity of adaptive immunity. These early studies of anti bodies 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).

Ilya Metchnikoff initially championed the cellular theory of immunity, which stated that host cells are the principal media tors of immunity. 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 prepare microbes for ingestion by phagocytes. These early cellularists were unable to prove that specific immunity to microbes could be mediated by cells. The importance of cellular immunity in host defense became firmly established in the 1950s, when it was shown that resistance to an intracellular bacterium, Listeria monocytogenes, could be transferred to animals with cells but not with serum. We now know that the specificity of cell-mediated immunity is due to T 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 an antigen is detectable only in individuals who have previously encountered the antigen, reflecting memory for that antigen. These individuals are said to be sensitized to the antigen, and the reaction is an indication of sensitivity. Such a reaction to a microbial antigen implies that the sensitized individual is capable of mounting a protective immune response to the microbe.

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