The immune systems of animals are capable of discriminating between cells sharing the same genotype as the host from cells with a different genotype. To do this, the express a set of proteins, often referred to as immune receptors that are capable of mediating this sort of recognition. One school of thought suggests that immune receptors may have evolved from Cell Adhesion Molecules (CAMs). The observation that some of the most ancient families of immune receptors play other roles lend credence to this notion.
The recogntion of non-self usually provokes some reaction in the host that results in the elimination of the foreign agent. The scale of the immunological response to the presence of foreign cells is related to the genetic distance between the host and the foreign tissue. Immune recognition of foreign cells falls into one of two categories: allorecognition (recognition of genetically similar (but not identical) cells, and xenorecognition (recognition of genetically unrelated cells).