Macrophages

mouse macrophage engulfing two particles (Wikipedia)

macrophages attacking a cancer cell (Wikipedia)

Macrophages are a type of white blood cell, so they are considered a part of the connective tissue. They have all the organelles of a typical eukaryotic cell and can be found residing in all tissues of the body, as well as circulating through the bloodstream. Macrophages have many different functions. When most people think of macrophages, they think of those cells that "eat" pathogens (foreign particles) that enter the body when one is hurt. However, they also play a large role in homeostasis, embryotic development, cell differentiation and wound repair. Because of their different roles, macrophages in different parts of the body can differ greatly. Their sizes can range from about 20 microns to 80 microns, and their lifespan can range for months to years.

3D rendering of a monocyte (Wikipedia)

Macrophages come from another type of white blood cell called monocytes, which are born from stem cells in the bone marrow. The monocytes mature into either resident macrophages that gather in specific areas that are more vulnerable to pathogens or into wandering macrophages, which travel through the bloodstream and the lymph vessels.

When tissue becomes damaged, monocytes move through the endothelium (wall) of blood vessels into the tissue and transforms into macrophages. These macrophages "eat" pathogens and other debris (like dead cells) in the damaged tissue through a process called phagocytosis. To do this, the macrophage extends its cell membrane around the particle it is engulfing and draws the particle into itself. Then it forms a vesicle called a phagosome around the particle and merges it with a lysosome, which releases enzymes that break down the captured particle. Since they cannot identify specific targets to attack, they are part of the innate immune response. They can only distinguish between body cells and foreign substances because body cells have a special protein coating that foreign substances do not have. They also aid the specific immune response by activating the adaptive (acquired) immune system. After a macrophage breaks down a foreign particle, it shows some of the broken-down proteins, called antigens, on its surface. Helper T cells can then read these antigens and identify which specific pathogens to target.

simplified diagram of phagocytosis (Wikipedia)

References:
Freitas, Robert A., Jr. "8.5.1 Cytometrics." Nanomedicine. N.p., 1999. Web. 31 Aug. 2015.

<http://www.nanomedicine.com/NMI/8.5.1.htm>.

Hume, David A. "The Macrophage Community Website." The Macrophage Community Website.

Roslin Institute, May 2012. Web. 31 Aug. 2015. <http://www.macrophages.com/>.

"Macrophage." New World Encyclopedia. MediaWiki, 2 Apr. 2008. Web. 31 Aug. 2015.

<http://www.newworldencyclopedia.org/entry/Macrophage>.

"Macrophages." Regenerative Medicine Partnership in Education. Duquese University, 2006.

Web. 31 Aug. 2015. <http://sepa.duq.edu/regmed/immune/macrophages.html>.