The scientific investigations reflected in this study present the aspects of identifying important mononuclear cells in the genesis of the immune system. In this investigation is presented the importance of lymphocyte determination by contacting heterologous erythrocytes resulting from couplings between major erythrocyte surface antigens and lymphocyte receptors. Important data reveals different possibilities of in vitro lymphocyte separation and cultivation, identification of surface specific markers of T and B lymphocytes. As a result of the separation of the mononuclear elements, relevant data on cellular activities were recorded by initiating the complex processes characteristic to the functions of the mononuclear cells involved in the immune response.