1.1.1　The Phagocytes

Phagocytes are the soldiers of the immune system, and provide innate immunity.They are responsible for swallowing, killing and digesting invading microbes.The process of swallowing microbes is known as phagocytosis.There are two main types of phagocyte.

· Microphages.These cells are also known as Polymorphonuclear Leucocytes, PMNs and Polymorphs.These cells start life in the bone marrow.They are constantly circulating in the blood.They cannot replicate, and live for only a few days.The bone marrow contains large reserves of microphages.

· Macrophages.These cells start out life as monocytes, which originate in the stem cells in the bone marrow, but when they are first called into action, they turn into macrophages.Macrophages are not as numerous as microphages, and there are no large reserves of them, but they are longer lived than microphages.Macrophages are stationed at strategic locations throughout the body,usually in places that are not otherwise well defended.These areas include the alveoli of the lungs,the abdominal (peritoneal) and chest (pleural) cavities, under the top layer of the skin and the intestines.Macrophages are the front line of defense against microbial invasion in these areas.

As mentioned above, the process of swallowing of microbes by the phagocytes is known as phagocytosis.After the invading microbe has been ingested, the next task for the phagocyte is to kill the microbe.This is achieved in two main ways.(https://www.daowen.com)

· Aerobically, i.e.using oxygen.The phagocytes produce oxygen based chemicals that are highly disruptive to the swallowed microbe.Oxygen is highly chemically reactive, and these oxygen based chemicals “tear” the microbe apart.This process is known as the oxidative burst, or the respiratory burst.

· Anaerobically, i.e.without using oxygen.One way to kill the microbe without oxygen is by using a chemical that deprives the microbe of iron, thus preventing it from metabolising.Another way is to increase the acidity of the internal environment of the phagocyte.

When these tasks are complete, the macrophages have one further task to complete.They return to the lymph nodes, displaying the remnants of the destroyed invader on their surface.This has the effect of stimulating the cells of the acquired immunity system into action.