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 adequately 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 immune response involving the T-cells, B-cells, and antibodies (immunoglobulins), and the nonspecific responses involving phagocytes and complement.
Unfortunately, some individuals have a weakened or absent immune system that 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.