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Primary immune deficiency diseases (PIDDs) are characterized by increased susceptibility to infections, often associated with autoimmunity and inflammation and an increased risk of malignancies because of impaired immune homeostasis and surveillance. Depending on the nature of the immune defect, the clinical presentation of PIDD may vary and may include recurrence of upper and lower respiratory tract infections, invasive bacterial and/or fungal infections, infections sustained by poorly virulent or opportunistic pathogens (Pneumocystis jirovecii, cytomegalovirus, environmental mycobacteria, Cryptosporidium spp., Giardia lamblia), persistent or recurrent candidiasis, narrow susceptibility to a selective type of pathogens, autoimmunity, and increased susceptibility to malignancies. Additional features can be seen in patients with defined immunodeficiency syndromes.

With the exception of immunoglobulin (Ig) A deficiency and DiGeorge syndrome, PIDDs are generally rare, with a prevalence of approximately 1 in 10,000 to 1 in 50,000 of the general population. However, prompt recognition of PIDD is important because diagnostic delay is associated with increased risk of death and irreversible complications. Most forms of PIDD follow Mendelian inheritance and present in childhood; however, some, such as common variable immunodeficiency, have a multifactorial origin and tend to present later in life.

The diagnostic approach to PIDD is based on a detailed family and clinical history, physical examination, and appropriate laboratory tests. Lymphopenia is characteristic of severe combined immune deficiency (SCID). Abnormalities affecting neutrophils can be observed in patients with disorders of neutrophil production (eg, congenital neutropenia, leukocyte adhesion deficiency; Chap. 63) or function (eg, chronic granulomatous disease; Chap. 64), respectively. Evaluation of serum Ig levels and of antibody responses to immunization antigens is of value for patients with a history of recurrent infections. The clinical presentation and the results of these screening evaluations may prompt additional laboratory testing. For instance, patients with a profound hypogammaglobulinemia and a history of recurrent infections should be tested for the presence of circulating B lymphocytes (CD19+ or CD20+ cells), which are absent or markedly reduced in X-linked agammaglobulinemia. On the other hand, early presentation with severe and/or opportunistic infections, especially if associated with lymphopenia, should prompt enumeration of lymphocyte subsets. A severe reduction of circulating CD3+ T cells is typically observed in SCID and may be associated with defects of B and/or natural killer cells. Deep bacterial infections and infections sustained by Aspergillus require evaluation of neutrophil count and function to identify patients with congenital neutropenia and chronic granulomatous disease, respectively. Invasive recurrent infections sustained by Neisseria spp. are an indication for assessing complement levels and function. The complement component deficiencies may also lead to systemic lupus erythematosus–like features or other autoimmune disorders. Laboratory results should be compared with age-matched control values because white blood cell counts, lymphocyte subsets, complement components, Ig levels, and antibody production (especially to polysaccharide antigens) undergo significant changes and progressive maturation in the first years of life. It is important to rule out secondary forms of immunodeficiency, such as HIV infection, protein loss, and immunodeficiency secondary to ...

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