Part 1 of Essay
is a group of cells, molecules, and tissues that help defend the body against diseases and other harmful invaders. 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 ....

Part 2 of Essay
.... by a body's immune system is called an antigen. Several types of cells may be involved in the immune response to antigens. When an antigen enters the body, it may be partly neutralized by components of the innate immune system. It may be attacked by phagocytes or by performed antibodies that act together with the complement system. The human immune system contains approximately 1 trillion T cells and 1 trillion B cells, located in the lymphoid organs and in the blood, plus approximately 10 billion antigen-pres ....