Normal human cells can also become foreign invaders. When an organ such as a liver is transplant (moved) form one person to another, cells from the transplanted organ are seen as enemy invaders by the immune system of the person receiving the organ. The cells of the transplanted organ set off an immunological attack. Killer T cells and antibodies rush to the site, causing organ rejection, and the organ dies. Physicians solve this problem in part with medicines that suppress, or slow down, the immune system.

　　正常的人体细胞也可能成为外来的入侵者，例如，一个人的肝脏器官移植到另一个人的身体内，接受者的免疫系统视来自被移植器官的细胞为入侵的敌人。被移植器官的细胞将发动一场免疫攻势，杀伤T细胞和抗体就会冲到现场，引起排斥反应，而且这个被移植的器官就会死亡。医生们可以使用抑制或减缓免疫系统的药物部分地解决这一难题。