Tumor cells may escape immune surveillance due to lack of costimulatory singal which is essential for the activation of host immune system. Effective activation of T lymphocytes requires two signals from antigen presenting cells (APCs).

• 肿瘤免疫学研究表明：肿瘤逃避机体免疫监视的机制之一可能是因为肿瘤细胞表面缺乏使T细胞完全活化的共刺激信号。