At the tissue site, red blood cells exchange oxygen for carbon dioxide and carry it back to the lungs to be exhaled. Each plays a distinct and important role in helping the immune system fight infection. Eosinophils attack protozoa that cause infection. Basophils are the least common type of white blood cell and their function is not completely understood. They play an important role in regulating allergic reactions such as asthma, hives, hay fever and reactions to drugs.
Monocytes are the largest white blood cells.
They engulf and destroy invading bacteria and fungi and clean up debris once foreign organisms have been destroyed by other white blood cells. When monocytes leave the bloodstream and enter tissues or organs, they can evolve into larger cells called macrophages that have an increased capacity to destroy foreign organisms invading the body. Lymphocytes are the smallest white blood cells and are the backbone of the immune system. Lymphocytes fight viral infections and assist in the destruction of other parasites, bacteria and fungi. One group of lymphocytes called T-cells regulates the immune system's response to invading organisms and is the body's main defense against viruses and protozoa.