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CD3 antigen, which is positive when stained with FITC-CD3. The latest advances in flow cytometry involve multiple-color analysis, enabling evaluation of coexpression of several markers while reducing the total amounts of antibody and specimen required for analysis. With the availability of multiple-color analysis, strategies have evolved that enable more refined gating of populations, such as CD45 gating for all WBCs or CD19 gating for B lymphocytes. This type of gating has enhanced or replaced the traditional scatter gating (Fig 1), with applications in immunodeficiency disorders, such as HIV infection,4 and hematologic neoplasia.5 In HIV infection, enumeration of CD4+ T cells has become a standard surrogate for estimating stage of disease.

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