In different congenital immunodeficiencies, the causative abnormality may be in components of the innate causative system, at different stages of lymphocyte development, or in the responses of mature lymphocytes to antigenic stimulation. Inherited abnormalities affecting innate immunity most commonly affect the complement pathway or phagocytes. Abnormalities in lymphocyte development may be caused by mutations in genes encoding a variety of molecules, including enzymes, adaptors, transport proteins, and transcription factors. These inherited defects, and the corresponding targeted disruptions in mice, have been useful for elucidating the mechanisms of lymphocyte development (see Chapter 8). Abnormalities in B lymphocyte development and function result in deficient antibody production and are diagnosed by reduced levels of serum immunoglobulin (Ig), defective antibody responses to vaccination, and, in some cases, reduced numbers of B cells in the circulation or lymphoid tissues or absent plasma cells in tissues (see Table 20-1). Abnormalities in T lymphocyte maturation and function lead to deficient cell-mediated immunity and may also result in reduced antibody production. Primary T cell immunodeficiencies are diagnosed by reduced numbers of peripheral blood T cells, low prolif-erative responses of blood lymphocytes to polyclonal T cell activators such as phytohemagglutinin, and deficient cutaneous delayed-type hypersensitivity (DTH) reactions to ubiquitous microbial antigens, such as Candida antigens. Defects in both humoral and cell-mediated immunity are classified under severe combined immunodeficiencies. In the following sections, we describe immunodeficiencies caused by inherited mutations in genes encoding components of the innate immune system or in genes required for lymphocyte development and activation. We conclude with a brief discussion of therapeutic strategies for these diseases.
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