The immune system
has two major functions:
- To recognize substances that are foreign to the body, and;
- To react to them.
An immune system that is functioning well may defend the body against infectious microorganisms (bacteria, viruses, fungi and parasites) and protect the body from the development of tumors and cancer. The mechanisms within the body that provide these defenses are the specific (adaptive) immune response involving immune cells (T cells, B cells, natural killer cells) and antibodies (immunoglobulins), and the nonspecific (innate) responses involving phagocytes and the complement system.
Unfortunately, some people have a weakened or absent immune system, a state we call immunodeficient or immunocompromised. In these people, the immune system is unable to recognize and react to foreign substances. The immune system's ability to kill invading microorganisms and limit the spread of infections may not be functioning well.