How Does the Immune System Kill Bacteria?
Macrophages Engulf Viruses and Bacteria in the Body
Aug 31, 2009
The first line of defense against bacteria, viruses and parasites that try to invade the body is the physical barrier. This includes the skin on the outer surface and the mucous membranes that line the digestive, respiratory and reproductive tracts – an area of almost 400 square meters, the size of two tennis courts.
The Innate Immune System
Bacteria and other foreign invaders do get through this first physical line of defense through cuts, scrapes and holes. The secondary protection inside the body is called the ‘innate immune system’ which all mammals naturally have.
Once bacteria enters the body through a cut or scrape in the skin or the respiratory or digestive system, the innate immune system kicks in. The area around the cut becomes red and inflamed as white blood cells move in to surround the trespassing bacteria. One type of white blood cells are called macrophages which can kill microbes by a process called phagocytosis. ‘Macro’ means 'large' and ‘phage’ comes from the Greek for ‘to eat” – exactly what this large cell does.
The macrophages recognize harmful substances in the body with antennae-like receptors and is usually the first to greet the unwelcome bacteria. The macrophage moves towards the bacterium by crawling and engulfs it in a pouch or vesicle called a phagosome. The macrophage then ‘swallows’ this pouch containing the microbe and fuses it with another vesicle called a lysosomes. Powerful digestive enzymes inside the lysosome ‘lyse’ or destroy bacteria.
What are Macrophages?
Macrophages and all other blood cells in the body are produced in the bone marrow, where they arise from self-renewing cells called stem cells. As the stem cells divide half of the daughter cells develop into the many types of blood cells, including erythrocytes (red blood cells) and white blood cells such as neutrophils and macrophages. When the cells that will become macrophages are first released from the bone marrow into the blood stream they are called monocytes. There are about two billion monocytes in the blood at a time and they travel through the capillaries into the tissues where they mature into macrophages and lay in wait for harmful invaders.
How Macrophages Affect the Immune System
Macrophages also work by triggering the innate immune system. When they phagocytose a bacterium, they also give off an alarm signal of chemicals which increase the flow of blood and fluid to the wound or area of infection causing redness and inflammation or swelling. These chemicals also stimulate nerves in the area causing pain.
Additionally macrophages produce important immune proteins called cytokines, which are hormones that deliver messages to other cells of the immune system and alert them to the infection site. This is called an “inflammatory response.”
This article is part of a series on How the Immune System Works. Read further to discover how antigens, antibodies, vaccines, vaccinations and other components of immunology function.
Reference:
Immunobiology - the immune system in health and disease, by Charles Janeway, Jr. and Paul Travers. Garland Publishing, Inc. Fifth edition, 2001.
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