Blast cells which were derived from rat lymphocytes by stimulation with phytohemagglutinin (PHA), concanavalin A (Con A), or pokeweed mitogen (PWM) transformed within 2–3 days into a new type of lymphocytes when plated without mitogen on embryo fibroblast monolayers. These lymphocytes were termed secondary lyrophocytes. Upon addition of PWM to PWM-secondary lymphocytes a marked adherence to fibroblast monolayers was observed. The degree of adherence was estimated (a) by direct count of the lymphocytes in the medium and in the trypsinized fibroblast fraction, and (b) by using 51Cr-labeled lymphocytes. The adherence process required incubation at 37°C. The process started immediately after the addition of PWM and reached a plateau at 6 hr. At this time more than 80% of the lymphocytes adhered. In the absence of PWM only 12% of the lymphocytes were found in the fibroblast fraction. Unlike PWM-lymphocytes. Con A-lymphocytes, PHA-lymphocytes, and ordinary lymphocytes taken directly from the rat lymph nodes adhered only slightly more in the presence of PWM (10–20% adherence of ordinary lymphocytes) than in its absence (8% adherence). The adherence of the secondary lymphocytes and the ordinary lymphocytes was also studied in the presence of Con A and PHA. These mitogens induced high rate of adherence and they did not demonstrate specificity in their action. The adherence was accompanied by transformation of the lymphocytes to blast cells endowed with target-cell lytic ability. This transformation occurred mostly in the adhering fraction of the lymphocyte population. The results support the notion that target-cell recognition and destruction in cellular immunity involve contact between the cells.

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