**6. The immune system and rejection**

The immune system is an evolutionarily ancient collection of structures, mechanisms and cells that detect and eliminate harmful organisms from the host. In older texts, the immune system is often described as distinguishing "self" from "non-self," but more recent research demonstrates that there are a variety of roles for the microbiome ("non-self" micro-organisms resident within, on or around the host) in maintaining the health of the host organism. Thus, the host immune system must be able to not only identify and eliminate harmful pathogens, it must also tolerate the presence of a variety of beneficial bacteria, fungi and yeast [21]. Because transplantation of cells, tissues and organs is an unnatural situation created through deliberate medical intervention, the human immune response uses incredible precision to identify even closely related human cells as "non-self" and efficiently removing them, a process referred to as "immune rejection." In general, the strength of the response is proportional to the degree of difference between the host and donor materials, therefore, when exposed to materials from an animal, the rejection response is much faster and stronger, increasing the challenge in controlling the immune response.
