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Definition

The ability of any given cell in the body to distinguish self from nonself is called the immune response.

All cells in the body are recognized as self. Any microorganism (for example, a foreign body or tumor) that invades or attacks the cells is recognized as nonself—or foreign—requiring the immune system to mount a combat against the nonself.

Immune system

The immune system is comprised of a network of immune cells that are generated in the bone marrow stem cell (a cell whose daughter cells may develop into other types of cells). From stem cells different types of immune cells originate that can handle specific immune functions. Phagocytes (cell eaters), serve as the first line of defense, engulfing dead cells, debris, virus, and bacteria. Macrophages...