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Common variable immunodeficiency (CVI) is a primary immunodeficiency characterized by deficient antibody production. The cause of this immunodeficiency is unknown; several in vitro studies have revealed a significant number of alterations that could explain the hypogammaglobulinemia present in this syndrome. Among those described are primary B cell alterations, numerical and functional T cell abnormalities, and defects in the interaction between accessory cells. The alteration typical of CVI is the failure of B lymphocytes to differentiate from antibody-producing cells, resulting in deficient immunoglobulin secretion. Among the T cell abnormalities described are a diminished proliferative response to mitogens and antigens, alterations in the level of production of several cytokines, especially reduction in the production of IL-2, diminished antigen-specific T cells and increase basal apoptosis after stimulation. Antigen presenting cells, monocytes and dendritic cells can also present alterations and contribute to deficient antigen response. The clinical manifestations of these patients is variable; most present recurrent bacterial infections due to encapsulated bacteria, especially sinusitis, otitis, bronchitis, and pneumonias. A few patients can present mycobacterial or fungal infection and occasionally Pneumocystis carinii. Viral infection is uncommon in these patients although some suffer recurrent herpes zoster infection. Clinical features of septicemia and central nervous system infections are less frequent. The incidence of digestive tract infections in these patients is high. The most common cause of diarrhea is Giardia lamblia; Salmonella, Shigella and Campylobacter are also common pathogens. Autoimmune disease is also more prevalent in these patients than in the general population. The most frequently associated diseases are hemolytic anemia, idiopathic thrombocytopenic purpura and autoimmune neutropenia. Cancer is also frequently associated with CVI, the most common forms being lymphoproliferative syndromes, especially non-Hodgkin's lymphoma. Granulomas are a unusual manifestation in some patients with CVI; their localization varies but the most commonly affected organs are the spleen and lungs. Some authors have compared these granulomas with those characterizing sarcoidosis, especially when appearing in the lung. Diagnosis of CVI is usually by exclusion of other diseases, such as cystic fibrosis, immotile cilia syndrome or allergic processes. CVI should be suspected in all patients with recurrent bacterial infections especially those localized in the respiratory tract. Other primary immunodeficiencies which present clinical findings similar to CVI and which should be ruled out are selective IgG subclass deficiency, IgA deficiency and selective deficiency in the response to polysaccharide antigens with normal immunoglobulin levels. The serum hypogammaglobulinemia present in all patients with CVI provides the diagnostic key. The age at which clinical manifestations appear, the absence of familial antecedents and the presence of circulating B lymphocytes form the basis of the differential diagnosis between X-linked agammaglobulinemia and autosomal recessive forms. The treatment of choice of patients with CVI is treatment with human gamma-globulin. Currently, the most common route of administration is intravenous; these molecules have a half-life of approximately 21 days and a high degree of safety concerning the possible transmission of viral infections. Adverse reactions are generally few and clinically unimportant. The most frequently used doses oscillate between 200 and 400 mg/kg body weight every 2-4 weeks. Both the dose and its frequency should be personalized for each patient. Early diagnosis of patients with CVI, application of treatment with appropriate antibiotics for infections and treatment with gamma-globulins prevent long-term complications of this disease and dramatically improve the quality of life and life expectancy of these patients.