The importance of this connection is increasing, however, with recent research findings in the field of immunology. The immune system comprises cells that recognize and destroy antigens, foreign substances that enter the body. These antigens can be bacteria, viruses, fungi, or parasites. When an antigen attacks, the cells of the immune system rally to respond. These white blood cells include lymphocytes, monocytes, macrophages, granulocytes, and mast cells.
Lymphocytes are the primary officers of the immune defense.
To defend the body, these immune system cells migrate to the antigen's location. The affected area becomes red, painful, and swollen, and the surrounding blood vessels congested. Lymphocytes secrete chemicals known as cytokines, which circulate throughout the blood to attract more cells to the scene of the crime. The most active lymphocyte is the T cell, so named because it develops in the thymus. These chemicals serve to calm down the inflammatory response after the offending antigen has been destroyed.