微嵌合体是指有一小部份数量，来自于另一个人、遗传上与宿主不同的细胞在宿主的身体内残留。这很可能是一种自体免疫的病，通常见于怀过孕的妇女，又或是她的子女身上发现；亦有可能是病人在接受了器官移植之后，有细胞从移植的器官游离并游走到宿主的身体其他地方。不过，具体的机制到现在还是不太清楚。

怀孕期间，由于母体及胎儿的免疫细胞有机会透过在胎盘进行的血液透析而进行交换，被交换的细胞在宿主内仍有可能会继续分裂，并在胎儿出生后的数十年后依然留在宿主体内。

Microchimerism (abbreviated Mc) is the presence of a small number of cells that originate from another individual and are therefore genetically distinct from the cells of the host individual. This phenomenon may be related to certain types of autoimmune diseases; however, the mechanisms responsible for this relationship are unclear.