Autoimmunity.

The immune system has evolved to protect an organism from the pathogens that invade it but the effector mechanisms involved in mediating this protection are potentially lethal to the host itself. Consequently it is essential that they are not elicited by the host's own tissues and, because biochemically self and non-self are very similar, the immune system has had to develop an exquisite capability to distinguish relatively minor differences. There has been considerable progress recently in understanding how this discrimination is achieved although many questions remain. The problem is important in that the mechanisms that ensure self tolerance occasionally fail. The consequences of this failure are the autoimmune diseases, many of which afflict Man. This article reviews what is known about the way that the immune system normally avoids self reactivity and how breakdown in self tolerance can occur.