NK cell abnormalities in cancer patients. A defective immunity secondary to tumor development has been well established in different types of cancer. The imbalance of immune status is inclined to immunosuppression in cancer patients which results in a concomitant tendency to tumor immune evasion. Such immunosuppression is characterized by a decrease in NK cell numbers in PB and a decreased in tumor infiltrate as compared to normal tissue. Moreover, in many types of cancer an altered phenotype which presents defective expression of activating receptors and overexpression of inhibitory receptors is observed.