Cells are the basic living units of tissue. The human body is composed of trillions of cells. There are over 200 different cell types, each specializes to perform essential functions. Bone cells form and regenerate the bones in the body. Nervous system cells send messages within the brain and throughout the body to produce actions. Cells in the blood carry oxygen and help the immune system. Male and female sex cells unite to produce offspring. The genetic information inside each type of cell acts as an instruction manual, telling a cell how to function and replicate.