Infections are the most frequent illness encountered in children by the pediatrician and, for most patients, represent normal life events. However, the child who has unduly frequent, persistent, or severe infections or who has infection(s) due to microbes that normally are nonpathogenic may represent the exceptional patient who has an immunologic defect. The causative organisms and the age at which the infections begin to occur may provide clues to the nature of the immunologic defect.

Components of the Immune System and Their Function in Host Defense

The immune system can be divided into two components based on the specificity with which microbes are discriminated from self. The innate or natural immune system is responsible for a rapid, efficient, and nonspecific response to invading microbes. It can be viewed as the first line of defense and is comprised of epithelial barriers, the spleen, phagocytic cells, natural killer (NK) cells, the complement system, and other proteins that aid with opsonization. Abnormalities in this part of the immune system generally predispose to infections from organisms that are ubiquitous and usually of low virulence. The second part of the immune system, the inducible or adaptive portion, is responsible for specific, amplifiable, and long-lived responses to foreign antigens.

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