The balance of the immune system. This cartoon represents the balance between the effector and regulatory functions of the immune system and emphasizes the role of effector clone size in dictating the outcome of an immune response. In the presence of large numbers of effector cells (red circles), the regulatory mechanisms, consisting of regulatory cells (green circles) and suppressive soluble factors such as cytokines, are unable to attenuate the effector’s response, which therefore predominates. In the context of transplantation this results in rejection. However, in the presence of low effector cell numbers these mechanisms can regulate the effectors and keep them in check. If these responses are sufficiently robust, transplant tolerance may result. Adapted from ref. 12.