INTRODUCTION

The principal function of the immune system is to prevent microbial infection. Therefore, disorders resulting in impaired function of the immune system (immunodeficiency) result in increased susceptibility to infection. Immunodeficiency can arise from an intrinsic defect of a component of the immune system (primary immunodeficiency, or PID). Alternatively, immunodeficiency may be secondary to another pathological condition, which adversely affects immune function (Table 5.1). Both primary and secondary immunodeficiencies result in increased susceptibility to infection. The precise pattern of infection depends on the specific component of the immune system that is affected. Most PIDs are caused by defects in single genes and are hence heritable. Others may represent the consequence of an interaction between the genetic phenotype and an environmental influence, like viral infections. Primary immunodeficiencies are rare and based on information from national registers; these diseases are estimated to occur between 1 in 2,000 to 1 in 10,000 live births. In contrast, secondary immunodeficiencies are more commonly seen in clinical practice. (See Table 5.1 for examples of secondary immunodeficiency.)

From a clinical perspective, immunodeficiencies can be classified into eight categories (Table 5.2). Each category has a characteristic pattern of clinical presentation (Table 5.3), which will be elaborated on later.

DEFECTS IN ANATOMICAL OR PHYSIOLOGICAL BARRIERS TO INFECTION

One of the commonest predisposing causes of infection is a defect in an anatomical or physiological barrier to infection. Intact epithelial membranes, especially stratified squamous epithelial surfaces such as the skin, constitute an extremely effective barrier to infection. Thus, integumentary damage caused by burns, eczema, and trauma (including surgery), predisposes to infection.