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 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-presenting cells located in the lymphoid
organs. To maximize the chances of encountering antigens wherever they may invade the
body, lymphocytes continually circulate between the blood and certain lymphoid tissues. A
lymphocyte spends an average of 30 minutes per day in the blood and recirculates about 50
times per day between the blood and lymphoid tissues.