Much of the current information describing the nature of cells and their interactions, indicates that there is a division of the immune system involving two lines of immunocompetent cells. According to our present knowledge, at least three types of cells are involved in the immune response. Two of these cells belong to the lymphocytic series and are classified as B-cells and T-cells. The third cell involved in this system is the phagocytic macrophage. This presentation will describe the events and mechanisms of cellular cooperation among these cell types which lead to the specific immune response.