The immune system is a group of cells, molecules, and tissues that help defend the
body against diseases and other harmful invaders. The immune system provides protection
against a variety of potentially damaging substances that can invade the body. These
substances include disease-causing organisms, such as bacteria, fungi, parasites, and
viruses. The body's ability to resist these invaders is called immunity. A key feature of the
immune system is its ability to destroy foreign invaders while leaving the body's own
healthy tissues alone. Sometimes, however, the immune system atta…
