Peripheral blood mononuclear cells (PBMC) can be used to assess cell-mediated immunity in general or, via antigen-specific stimulation, to detect previous exposure to a variety of antigens/allergens and to monitor the response to immunotherapies. Peripheral blood is the most common source of mononuclear cells for in vitro cultures, although mononuclear cells can be obtained from other sources involved in the allergic reaction. PBMC from individuals previously exposed to an antigen proliferate in vitro when stimulated with the specific antigen. Proliferation is measured by the incorporation of (³H)-thymidine into newly synthesized DNA. This parameter is often used as an end point of lymphocyte stimulation induced by antigen or antigen fragments (e.g., synthetic peptides), mitogens, or anti-CD3/anti-CD28 combinations.