Phytohemagglutinin causes cultured lymphocytes to agglutinate, divide, and produce γ-globulin. Most cells are transformed into large lymphocytes, some resembling plasmocytes. Actinomycin D stops γ-globulin production after 2 hours. When specific antigens are added to lymphocytes from sensitized individuals, only some cells undergo morphological transformation, produce γ-globulin, and divide. When cells or cell extracts from an unrelated individual are added to a culture, a similar reaction occurs.