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Tumors, both malignant and benign, “present” (first become observable) as lumps or masses caused by the abnormal growth of cells. Many benign tumors are encased in a well-formed capsule. Malignant tumors, on the other hand, lack a true capsule and, even when limited to a specific location, invariably can be seen to have infiltrated surrounding tissues. The ability to invade adjacent tissues is a major characteristic that distinguishes malignant tumors from benign tumors. A tumor mass is composed not only of abnormal tumor cells but also of normal host cells, which nourish the tumor, and immune cells, which attempt to ...(100 of 22022 words)