Even though development of tumors is not common in all individuals, formation of tumor cells often takes place in almost all individuals. But the efficient surveillance of immune system identifies the abnormal cells whenever they develop inside the body and removes them before the formation of tumor.
Tumor in certain individuals indicates either failure of defense mechanism or improper function of immune system. Chances of cancer development is high in patients with suppressed or week immune activity. Patients with AIDS are susceptible for various types of tumors.
In normal conditions immune response to tumors is similar to the immune response shown against foreign grafts. It may be specific like the response against a foreign cell, virus infected cell etc. or non-specific with the involvement of macrophages and natural killer cells like innate immune response to various foreign agents.