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.
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