Congenital immunodeficiencies that affect both humoral and cell-mediated immunity are called combined immunodeficiencies, and a subset of these in which most peripheral T cells are missing or defective are known as severe combined immunodeficiencies (SCIDs) (Table 20-3). These diseases are characterized by deficiencies of both B and T cells or only of T cells; in the latter cases, the defect in humoral immunity is due to the absence of T cell help. Children with SCID usually have infections during the first year of life, Pneumocystis jiroveci pneumonia being particularly common, and they succumb to these infections unless they are treated.
SCID results from impaired T lymphocyte development with or without defects in B cell maturation (Fig. 20-1). The thymic epithelium contributes in a major way to early T cell development. The process of T (and B) lymphocyte maturation from hematopoietic stem cells to functionally competent mature lymphocytes involves proliferation of early lymphocyte progenitors, rearrangement of the locus encoding one chain of the antigen receptor followed by selection of cells that have made in-frame productive rearrangements at a pre-antigen receptor checkpoint, expression of both chains of the antigen receptor, and selection of cells with useful specificities (see Chapter 8). Defects in many of these steps
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