Macrophages

Germ-Eating White Blood Cells

Macrophage Fighting Bacteria
This image shows a macrophage cell capturing bacteria. Macrophages are white blood cells that engulf and digest pathogens. Credit: Science Picture Co/Collection Mix: Subjects/Getty Images

Macrophages

Macrophages are immune system cells that are vital to the development of non-specific defense mechanisms that provide the first line of defense against pathogens. These large immune cells are present in nearly all tissues and actively remove dead and damaged cells, bacteria, cancerous cells, and cellular debris from the body. The process by which macrophages engulf and digest cells and pathogens is called phagocytosis.

Macrophages also aid in cell mediated or adaptive immunity by capturing and presenting information about foreign antigens to immune cells called lymphocytes. This allows the immune system to better protect against future attacks from the same invaders. In addition, macrophages are involved in other valuable functions in the body including hormone production, homeostasis, immune regulation, and wound healing.

Macrophage Phagocytosis

Phagocytosis allows macrophages to get rid of harmful or unwanted substances in the body. Phagocytosis is a form of endocytosis in which matter is engulfed and destroyed by a cell. This process is initiated when a macrophage is drawn to a foreign substance by the presence of antibodies. Antibodies are proteins produced by lymphocytes that bind to a foreign substance (antigen), tagging it for destruction. Once the antigen is detected, a macrophage sends out projections which surround and engulf the antigen (bacteria, dead cell, etc.) enclosing it within a vesicle.

The internalized vesicle containing the antigen is called a phagosome. Lysosomes within the macrophage fuse with the phagosome forming a phagolysosome. Lysosomes are membranous sacs of hydrolytic enzymes formed by the Golgi complex that are capable of digesting organic material. The enzyme content of the lysosomes is released into the phagolysosome and the foreign substance is quickly degraded.

The degraded material is then ejected from the macrophage.

Macrophage Development

Macrophages develop from white blood cells called monocytes. Monocytes are the largest type of white blood cell. They have a large, single nucleus that is often kidney-shaped. Monocytes are produced in bone marrow and circulate in the blood anywhere from one to three days. These cells exit blood vessels by passing through blood vessel endothelium to enter into tissues. Once reaching their destination, monocytes develop into macrophages or into other immune cells called dendritic cells. Dendritic cells aid in the development of antigen immunity.

Macrophages that differentiate from monocytes are specific to the tissue or organ in which they reside. When the need for more macroghages arises in a particular tissue, the residing macrophages produce proteins called cytokines that cause responding monocytes to develop into the type of macrophage needed. For example, macrophages fighting infection produce cytokines that promote the development of macrophages that specialize in fighting pathogens. Macrophages that specialize in healing wounds and repairing tissue develop from cytokines produced in response to tissue injury.

Macrophage Function and Location

Macrophages are found in almost every tissue in the body and perform a number of functions outside of immunity. Macrophages aid in the production of sex hormones in male and female gonads. Macrophages assist in the development of blood vessel networks in the ovary, which is vital for the production of the hormone progesterone. Progesterone plays a critical part in the implantation of the embryo in the uterus. In addition, macrophages present in the eye help to develop blood vessel networks necessary for proper vision. Examples of macrophages that reside in other locations of the body include:

  • Adipose Tissue - Macrophages in adipose tissue protect against microbes and also help adipose cells to maintain the body's sensitivity to insulin.
  • Integumentary System - Langerhans cells are macrophages in the skin that serve an immune function and aid in the development of skin cells.
  • Kidneys - Macrophages in the kidneys help to filter microbes from blood and aid in the formation of ducts.
  • Spleen - Macrophages in the red pulp of the spleen help to filter damaged red blood cells and microbes from blood.
  • Lymphatic System - Macrophages stored in the central area (medulla) of lymph nodes filter lymph of microbes.
  • Reproductive System - Macrophages in gonads aid in sex cell development, embryo development, and the production of steroid hormones.
  • Digestive System - Macrophages in the intestines monitor the environment protecting against microbes.
  • Lungs - Macrophages present in the lungs, known as alveolar macrophages, remove microbes, dust, and other particles from respiratory surfaces.
  • Bone - Macrophages in bone may develop into bone cells called osteoclasts. Osteoclasts help to break down bone and to reabsorb and assimilate bone components. Immature cells from which macrophages are formed reside in non-vascular sections of the bone marrow.

Macrophages and Disease

Although a primary function of macrophages is to protect against bacteria and viruses, sometimes these microbes can evade the immune system and infect immune cells. Adenoviruses, HIV, and the bacteria that cause tuberculosis are examples of microbes that cause disease by infecting macrophages.

In addition to these types of diseases, macrophages have been linked to the development of diseases such as heart disease, diabetes, and cancer. Macrophages in the heart contribute to heart disease by aiding in the development of atherosclerosis. In atherosclerosis, artery walls become thick due to chronic inflammation induced by white blood cells. Macrophages in fat tissue can cause inflammation which induces adipose cells to become resistant to insulin. This can lead to the development of diabetes. Chronic inflammation caused by macrophages can also contribute to the development and growth of cancer cells.

Sources:

  • White Blood Cells. The Histology Guide. Accessed 09/18/2014 (http://www.histology.leeds.ac.uk/blood/blood_wbc.php)
  • The Biology of Macrophages - An Online Review. Macrophage Biology Review. Macrophages.com. Published 05/2012 (http://www.macrophages.com/macrophage-review)
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Bailey, Regina. "Macrophages." ThoughtCo, Feb. 6, 2017, thoughtco.com/macrophages-meaning-373352. Bailey, Regina. (2017, February 6). Macrophages. Retrieved from https://www.thoughtco.com/macrophages-meaning-373352 Bailey, Regina. "Macrophages." ThoughtCo. https://www.thoughtco.com/macrophages-meaning-373352 (accessed December 18, 2017).