A patient's blood sample is incubated with an antigen and tested for lymphocyte response, ie. an activation of lymphocytes and/or a conversion of lymphocytes to lymphoblasts, which indicates prior exposure of the patient to the antigen. A positive response indicates the presence of prior exposure to prior diseases or clinical conditions such as parasitic diseases, tuberculosis, salmonellosis, gonorrhea, fungal infections, rickettsial infections, Lyme disease or allergens. Whole blood from the patient is incubated with the antigen of the disease or condition for which the patient is being tested. After a suitable time, a fluorescent dye or colorant is added which has an affinity for a discriminant characteristic of the activated lymphocytes or lymphoblasts, such as: intracellular calcium; surface activation antigens such as transferrin receptor; HLA-Dr; Leu-23; and the like. The incubated blood is then drawn into a transparent tube containing a float which concentrates the buffy coat constituent layers upon centrifugation of the blood sample. The concentrated lymphocyte layer is then examined for fluorescence or coloration which is indicative of the presence of the activated lymphocytes or lymphoblasts, and their concentration. The fluorescence or coloration can be qualified and/or quantified by a reader instrument.