The Immune System

The Immune System

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 attacks and damages these healthy tissues.
This reaction is called an autoimmune response or autoimmunity.

The immune system is composed of many parts that work together to fight infections when pathogens or poisons invade the human body. Pathogens are disease-causing organisms such as bacteria and viruses. The immune system reacts to foreign substances through a series of steps know as the immune response. Any agent perceived as foreign 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 cont...