| Abstract: | The role of mitogens during lymphocyte activation was studied with kidney bean leucoagglutinin, concanavalin A and kidney bean phytohemagglutinin. The mitogens were removed by treatment with appropriate antisera, which was demonstrated to remove also mitogens already attached to the cells. The process of lymphocyte activation in vitro can be divided into four distinct steps, three of which are mitogen-dependent and the fourth is mitogen-independent. The first step consists of attachment of the stimulatory molecules to the cell membrane. The second step consists of reaction between mitogen and an activating system. During these two phases the cells become preactivated. The establishment of a preactivated state involves at least some synthesis of cytoplasmic RNA. The preactivated state is reversible and during the third step of lymphocyte activation the final result of preactivation is determined. Depending on the presence or absence of mitogen the cells may remain preactivated for over 60 h, they may return to the resting state or they may proceed through the final stages of the proliferation cycle and eventually divide. This fourth step is independent of the presence or absence of mitogen. A prolonged contact between cells and mitogen is required during the mitogen-dependent steps. The process of lymphocyte activation by mitogen is thus continuously being regulated by the stimulatory molecules on the lymphocyte membrane, which may be of considerable significance also for in vivo immunologicai reactions at the cellular level. |
