It is clear by now that cell-to-cell interactions involving a variety of signals are required for effective immune response. The data reviewed here suggest that CD40-CD40L interactions are critical for development of CD4 T-cell-dependent effector functions. Lack of this important interaction results in greatly reduced activation of CD4 T cells, while successful interaction of these molecules results in full activation of these T cells. Consequently, the absence of CD40-CD40L interactions leads to impairment of T-cell effector such as help for B-cell differentiation and class switch, activation of monocytes and macrophages to produce cytokines and to kill intracellular pathogens, and activation of autoreactive T cells to mount an autoimmune response. The effector functions of T cells controlled by CD40-CD40L interactions in a successful immune response are given in Table I. Data presented so far suggest that CD40-CD40L interactions play a role in early signalling events, where interactions of this kind are required to induce expression of costimulatory molecules on APC. One possible sequence of events in that APC, like DC, take up antigens at the site of injury or infection and migrate to lymph nodes, where they present antigens complexed with MHC class II molecules to naive T cells. This results in expression of CD40L on T cells. Coupling of this newly expressed CD40L on T cells with CD40 on APC results in expression of the costimulatory activity of the APC. At this time the costimulatory signal provided by the APC is received by the T cells via CD28/CTLA-4, which drives the cell to enter into cell cycle and complete T cell activation. T cells thereby activated can now enter into secondary cognate CD40-CD40L-dependent effector recognition with B cells to switch Ig class, macrophages to produce cytokines and new DC carrying the same antigen to up-regulate costimulatory activity. A tight regulation of expression of CD40L on T cells and costimulatory activity on APC would prevent activation of unwanted bystander T cells. The coupling of activation of the APC primed with the cognate antigen to the activation of the T-cell specific for that antigen in this model provides an additional regulatory step in the initiation of the immune response. This also suggests that a limited number of T cells/APC will be activated, both of which will be specific in nature. This additional step may be important for safeguarding against an autoimmune response. In addition, the fact that CD40L uniquely seems to play this role suggests that selective immunotherapies to treat autoimmune disease and prevent graft rejection can be targeted on this molecule. On the other hand, CD40-directed approaches to up-regulate costimulatory activity on APC could be developed to fight tumor growth, contain infections and treat immunodeficiencies.