Escape from host-antitumor immunity

Cancer cells may express proteins recognizable by the individual's immune system as foreign because they are either tumor-specific or not expressed at high levels in normal tissues to which the host is tolerant. There is now much evidence that tumors can be immunogenic, that is, that they frequently express antigens in a form recognizable by the host immune system. This has been shown not only in experimental animals but also for spontaneously occurring human tumors. Tumors therefore may progress by evolving variants that can evade immune responses or by developing other strategies to "escape" the immune response. The purpose of this review is to consider the current status of knowledge concerning these different tumor escape strategies.