Use of High Dose X‐Irradiation to Block Back Stimulation in the MLC Reaction

Paradoxical stimulation, “back stimulation” was observed in MLR (mixed lymphocyte culture reaction) in both family and population studies. This is one of the major problems in obtaining clear cut‐off points for stin ulation and non‐stimulation in MLR using LD (lymphocyte defined) homozygous typing cells. The ability to provoke back stimulation was found to be different among LD homozygous typing cells. The presence of nonspecific blastogenic factors in supernatant from mixed culture of LD homozygous cells and heterozygous cells, which might be responsible for back stimulation, was confirmed. It was clearly shown that irradiation of LD homozygous typing cells with 6,000 rads instead of the widely used 3,000 rads can greatly reduce or eliminate this back stimulation without introducing any false non‐stimulation.