The immune response protects individuals against infections. In order to do this effectively the immune system must be able to distinguish between invading organisms and its own host, attacking the former whilst leaving the latter unharmed. This is not such an easy matter given that invading organisms occur in many forms, as viruses, bacteria, yeasts, worms and so on and that the invaders are constantly mutating to avoid attack by their potential hosts. Given the difficulty of the task it is not surprising that the immune system has evolved many ways to distinguish between invaders and its own host. The mechanisms used fall into two classes. One class involves recognition of features of the invader which are not present on the host. If this type of recognition occurs, the target is destroyed. Another class involves recognition of features of the host which are not present on the invader. If this type of recognition occurs, the immune response is inhibited and the target is not destroyed. Thus the immunologic self is defined in two ways, either by the absence of something foreign and/or by the presence of something familiar.