The cells of the body have varied and essential roles. Skin cells protect the whole body; red blood cells carry oxygen; and bone cells build the skeleton. Cells multiply fast when a person is growing, but their activity stabilizes with maturity. In grown-ups, cells divide and renew themselves to replace dead tissue or to help in the healing process. Cancerous cells are runaway cells that multiply rapidly and without obvious purpose. They destroy healthy tissue and tend to spread at an alarming speed. Normal cell reproduction is important .