The collaborative character of the immune response requires direct cell- to cell or humoral contacts between the lymphocytes and the cells of mononuclear phagocyte system. The proposed model of direct interrelationship between the cells of the CNS and the immune system throws much light on the morphology of the lymphocyte entry and its recruitment into parenchyma during pathology. The structure, distribution and membrane morphology of the formed in situ rosette-like clusters (containing microglia, as CNS type of mononuclear phagocytes, and syngeneic lymphocytes) was investigated by light and scanning electron microscopy (SEM). The results were discussed in relation with the strong expression of a number of immune capacities from the ubiquitiously situated in the CNS microglia and the pivotal role of the lymphocytes and microglia in pairs with a close contact to each other in the brain immune processing.