Complement assays include the CH50 test, which measures the presence of proteins in the classical pathway of complement(C1, C2, C3, C4), and the AH50 test, which tests the proteins of the alternative pathway of complement (C3, factor B, proper-din). If both the CH50 and AH50 levels are abnormal, a defect in the common pathway is likely (C5−C9). Specialized laboratories can measure the presence or function of specific complement proteins. Tests for C1-inhibitor antigen and function areused to diagnose hereditary or acquired angioneurotic edema.
Tests for neutrophil function include the nitroblue tetrazolium (NBT) or dihydrorhodamine 123 (DHR) test for CGD,in which oxygen radicals generated by activated neutrophilsoxidize NBT to an insoluble blue dye and DHR to a fluorescentmolecule. Patients with CGD have no blue-staining neutrophilswith NBT and little fluorescence when DHR is used. In vitro tests for evaluation of neutrophil phagocytosis, chemotaxis,bacterial killing, and for the presence of myeloperoxidase activity are available in some laboratories. Tests for the expression ofadhesion molecules such as CD18 (leukocyte function–associated antigen type 1, LFA-1) can be performed by flow cytometry.
Genetic testing to confirm the diagnosis of a primary immunodeficiency disease can be performed in specialized laboratories and may be helpful for deciding on a course of treatment, determining the natural history and prognosis of the disease, genetic counseling, and prenatal diagnosis. In patients in whom DiGeorge syndrome is suspected, fluorescent in situ hybridization studies for deletions of chromosome 22 can be helpful. In patients in whom ataxia-telangiectasia is suspected, chromosomal studies for breakage in chromosomes 7 and 14 are useful.
The absence of a thymus on chest x-ray suggests DiGeorge syndrome or other defects in T-cell development. Abnormalities in the cerebellum are found in patients with ataxia-telangiectasia. Otherwise the use of diagnostic imaging in the evaluation of immunodeficiency diseases is essentially limited to the diagnosis of infectious diseases.
Disorders that affect lymphocyte development or function result in significant immunodeficiency because lymphocytes provide antigen specificity and memory responses. Hematopoietic stem cells give rise to lymphoid precursors that develop into T lymphocytes in the thymus or B lymphocytes in the bone marrow (Fig. 73-1). Isolated B-cell disorders result in antibody deficiency diseases, whereas T-cell disorders usually cause combined immunodeficiency because they are necessary for cell-mediated immunity to clear intracellular pathogens and for antibody synthesis by B cells. NK cells are an important component of the innate immune response, develop from lymphoid precursors, and can kill virus-infected cells and tumor cells. Antibodies can enhance NK cell function by antibody-mediated cellular cytotoxicity.
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Disorders of B cells result in an increased susceptibility to infections by encapsulated bacteria.