Lymphoid Immunodeficiencies May Involve B Cells T Cells or Both

The combined forms of lymphoid immunodeficiency affect both lineages and are generally lethal within the first few years of life; these arise from defects early in developmental pathways. They are less common than conditions, usually less severe, that result from defects in more highly differentiated lymphoid cells.

B-cell immunodeficiency disorders make up a diverse spectrum of diseases ranging from the complete absence of mature recirculating B cells, plasma cells, and immuno-globulin to the selective absence of only certain classes of immunoglobulins. Patients with these disorders usually are subject to recurrent bacterial infections but display normal immunity to most viral and fungal infections, because the T-cell branch of the immune system is largely unaffected. Most common in patients with humoral immunodeficiencies are infections by such encapsulated bacteria as staphylococci, streptococci, and pneumococci, because antibody is critical for the opsonization and clearance of these organisms.

Because of the central role of T cells in the immune system, a T-cell deficiency can affect both the humoral and the cell-mediated responses. The impact on the cell-mediated system can be severe, with a reduction in both delayed-type hypersensitive responses and cell-mediated cytotoxicity.

- X-linked chronic granulomatous disease (CGD) Properdin deficiency Wiskott-Aldrich syndrome (WAS)

X-linked severe combined immunodeficiency

X-linked agammaglobulinemia (Bruton's tyrosine kinase)

X-linked hyper-IgM syndrome (XHM)

FIGURE 19-2

Several X-linked immunodeficiency diseases result from defects in loci on the X chromosome. [Data from the Natl. Center for Biotechnology Information Web site]

Immunoglobulin deficiencies are associated primarily with recurrent infections by extracellular bacteria, but those affected have normal responses to intracellular bacteria, as well as viral and fungal infections. By contrast, defects in the cellmediated system are associated with increased susceptibility to viral, protozoan, and fungal infections. Intracellular pathogens such as Candida albicans, Pneumocystis carinii, and Mycobacteria are often implicated, reflecting the importance of T cells in eliminating intracellular pathogens. Infections with viruses that are rarely pathogenic for the normal individual (such as cytomegalovirus or even an attenuated measles vaccine) may be life threatening for those with impaired cell-mediated immunity. Defects that cause decreased T-cell counts generally also affect the humoral system, because of the requirement for TH cells in B-cell activation. Generally there is some decrease in antibody levels, particularly in the production of specific antibody after immunization.

As one might expect, combined deficiencies of the humoral and cell-mediated branches are the most serious of the immunodeficiency disorders. The onset of infections begins early in infancy, and the prognosis for these infants is early death unless therapeutic intervention reconstitutes their defective immune system. As described below, there are increasing numbers of options for the treatment of immunodeficiencies.

The immunodeficiencies that affect lymphoid function have in common the inability to mount or sustain a complete immune response against specific agents. A variety of failures can lead to such immunodeficiency. Defective intercellular communication may be rooted in deleterious mutations of genes that encode cell-surface receptors or signal-transduction molecules; defects in the mechanisms of gene rearrangement and other functions may prevent normal B- or T-cell responses. Figure 19-3 is an overview of the molecules involved in the more well-described interactions among T cells and B cells that give rise to specific responses, with a focus on proteins in which defects leading to immunodeficiency have been identified.

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