THERE have been several reports of the in vivo effects of phytohaemagglutinin (PHA) on the primary immune response, but the conclusions are equivocal and the mode of action of PHA is still far from clear. Elves1 has recently shown that PHA acts as an immunosuppressor exclusively on the production of humoral antibody, provided both the PHA and the antigen are given intraperitoneally. Apart from this special case2-6, the effect of PHA on immunological reactions has been stimulatory rather than suppressive7-10. Histological examinations11-13 and diffusion chamber experiments3 have shown that in vivo treatment with PHA results in a pronounced increase in the number of immunoblast-like cells. Nevertheless it is not yet clear whether this effect has any relationship to the immunological results. We therefore decided to determine whether PHA has an effect on the response of mice to heterologous erythrocytes, as judged by their ability to produce the plaque forming cells using the Jerne technique, and to investigate the morphology of the antibody producing cells observed.
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