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Chapter 68: Immunodeficiency

INTRODUCTION

Immunodeficiency can occur in any of the four major components of the immune system: (1) B cells (antibody), (2) T cells, (3) complement, and (4) phagocytes. In a patient with a history of infections that are unusually frequent, unusually severe, or caused by unusual organisms, the pattern of these infections can indicate which component(s) of the immune system might be defective. Most immunodeficiencies are acquired, and these are frequently caused by immunosuppressive medications or diseases that suppress immunity, such as HIV/AIDS. Although they are less common, congenital immunodeficiencies (Table 68–1) are important to understand because (1) the patterns of infections that are seen teach us how various immune components are supposed to function normally, and (2) recent technological advances have allowed us to better diagnose and treat these diseases, preventing the infectious complications.

TABLE 68–1Important Congenital Immunodeficiencies
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TABLE 68–1 Important Congenital Immunodeficiencies
Deficient Component and Name of Disease Specific Deficiency Molecular Defect Clinical Features
Combined B and T cell
Severe combined immunodeficiency (SCID) Deficiency of both B-cell and T-cell function Various mutations: Defective interleukin (IL)-2 receptor, defective recombinases, defective kinases, absence of class II MHC proteins, or ADA or PNP deficiency Bacterial, viral, fungal, and protozoal infections
T cell      
Thymic aplasia (DiGeorge’s syndrome) Absence of T cells and suppressed antibody responses Defective development of pharyngeal pouches, associated with chromosome 22 deletions Viral, fungal, and protozoal infections; tetany caused by hypoparathyroidism
Chronic mucocutaneous candidiasis Deficient T-cell response to Candida IL-17 and IL-17 receptor deficiencies have been described Skin and mucous membrane infections with Candida
B cell      
X-linked (Bruton’s agammaglobulinemia) Absence of B cells; very low immunoglobulin (Ig) levels Mutant tyrosine kinase Recurrent bacterial infections, especially of respiratory tract, caused by pyogenic bacteria such as pneumococci
Selective IgA Very low IgA levels Failure of heavy-chain gene switching Recurrent infections, especially of the sinuses and lung, caused by pyogenic bacteria
Complement      
C3b Insufficient C3 Unknown Pyogenic infections, especially with Staphylococcus aureus
C6,7,8 Insufficient C6,7,8 Unknown Neisseria infections
Phagocytes      
Chronic granulomatous disease Defective bactericidal activity because no oxidative burst Deficient NADPH oxidase activity Pyogenic infections, especially with S. aureus and Aspergillus

ADA = adenosine deaminase; MHC = major histocompatibility complex; NADPH = nicotinamide adenine dinucleotide phosphate hydrogen; PNP = purine nucleoside phosphorylase.

CONGENITAL IMMUNODEFICIENCIES

T-Cell Deficiencies and Combined T- and B-Cell Deficiencies

Congenital T-cell deficiencies tend to be the most severe and easily recognized immunodeficiency. Because T cells are central to so many aspects of immune responses, including antiviral immunity and the activation of macrophages, their absence results in a broad range of unusual opportunistic infections (i.e., viruses, bacteria, fungi, and protozoa that are rarely seen in healthy hosts). In addition, because T cells are required for maturation of B ...

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