字面上講是指不同組織共存的能力；嚴格地講是指所有移植蛋白的一致性，這是阻止移植和器官排斥的需要。組織相容性的分子基礎是修飾幾乎所有人類細胞表面的一套移植蛋白。這些蛋白是由位于6號染色體上的一段稱為主要組織相溶性復合體基因，MHC編碼的。這些蛋白高度多態。例如，它們在不同的人中顯示差異。盡管很多人會有一些相同的MHC分子，極少數人有完全相同的MHC分子。微小的差別導致這些蛋白質被移植受體的免疫系統識別為外來的而進行破壞。對成功的移植來說這些蛋白質應該在供體和受體之間相匹配。雙胞胎相配的幾率最高，接下來是兄弟姐妹。在一般人群中只有10萬分之一的比例是MHC匹配的，可以允許移植。

Literally, the ability of different tissues to “get along”; strictly, identity in all of the transplantation proteins, which is a requirement for the prevention of graft or organ rejection. The molecular basis of histocompatibility is a set of transplantation proteins that decorate the surface of nearly all human cells. These proteins are encoded by genes that are grouped on a part of chromosome 6 called the major histocompatibility complex, or MHC. These proteins are highly “polymorphic” i.e., they show variation in different individuals. Although many individuals may share some identical MHC molecules, a very low number share all the MHC molecules. The consequence of these minor differences is that these proteins are recognized by the transplant recipient’s immune system as being foreign, and so are targeted for destruction (since the immune system’s job is to eradicate any foreign proteins or cells that invade the body). For successful transplantation these proteins ideally should be matched between donor and recipient. Twins have the highest rate of match, followed by siblings. In the general population only 1 in 100,000 individuals is sufficiently “MHC matched” to another person to allow transplantation.