V

Serum (antibodies)

Block infections and eliminate extracellular microbes

Cell-mediated immunity

Phagocytosed microbes in macrophage

Helper T lymphocyte

Helper T lymphocyte

Cells (T lymphocytes)

Activate macrophages to kill phagocytosed microbes

Intracellular microbes (e.g., viruses) replicating within infected cell

Cytotoxic ยป T lymphocyte

Cells (T lymphocytes)

Kill infected cells and eliminate reservoirs of infection mediated by different components of the immune system and function to eliminate different types of microbes (Fig. 1-2). Humoral immunity is mediated by molecules in the blood and mucosal secretions, called antibodies, which are produced by cells called B lymphocytes (also called B cells). Antibodies recognize microbial antigens, neutralize the infectivity of the microbes, and target microbes for elimination by various effector mechanisms. Humoral immunity is the principal defense mechanism against extracellular microbes and their toxins because secreted antibodies can bind to these microbes and toxins and assist in their elimination. Antibodies themselves are specialized and may activate different effector mechanisms. For example, different types of antibodies promote the ingestion of microbes by host cells (phagocytosis), bind to and trigger the release of inflammatory mediators from cells, and are actively transported into the lumens of mucosal organs and through the placenta to provide defense against ingested and inhaled microbes and against infections of the newborn, respectively. Cell-mediated immunity, also called cellular immunity, is mediated by T lymphocytes (also called T cells). Intracellular microbes, such as viruses and some bacteria, survive and proliferate inside phagocytes and other host cells, where they are inaccessible to circulating antibodies. Defense against such infections is a function of cell-mediated immunity, which promotes the destruction of microbes residing in phagocytes or the killing of infected cells to eliminate reservoirs of infection.

Protective immunity against a microbe is usually induced by the host's response to the microbe (Fig. 1-3). The form of immunity that is induced by exposure to a foreign antigen is called active immunity because the immunized individual plays an active role in responding to the antigen. 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 exposures to that microbe are said to be immune.

Immunity can also be conferred on an individual by transferring serum or lymphocytes from a specifically immunized individual, a process known as adoptive transfer in experimental situations (see Fig. 1-3). The

Active immunity

Days or weeks

Microbial antigen (vaccine or infection)

Specificity Memory

Challenge infection

Challenge infection

Days or weeks

Serum (antibodies) from immune individual

Infection

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