Macrophages

Macrophages are located throughout the body and function as professional phagocytic cells that ingest and process foreign materials and cell debris. They act not only as part of the first line of defense against invading pathogens, but they are also involved in maintaining tissue integrity and homeostasis. Additionally, macrophages may have specialized functions, depending on their tissue of residence and gene expression profiles. Therefore, macrophages are considered to be a highly heterogeneous cell population that can rapidly change their phenotypes and functional properties based on signals in their extracellular environment. When homeostatic conditions are disrupted, signals in the extracellular environment can alter the transcriptional program of a macrophage and lead to an activated state. Several phenotypically different macrophage activation states have been characterized that have different functional properties, including pro-inflammatory M1 macrophages and anti-inflammatory M2a, M2b, M2c, and M2d macrophages. Based on differences in their phenotypes, each of these activated states can be identified based on the expression or lack of expression of certain cell surface and intracellular markers. Abnormalities in macrophage functions have been associated with a wide range of chronic inflammatory and autoimmune diseases.