Macrophages and Monocytes

Macrophages and monocytes are cells of the adaptive immune system which serve to clean up debris via phagocytosis. They are mononuclear phagocytes which survey their local tissue system to ensure cells and ECM components show the normal and healthy markers associated with homeostatic balance. Variations or disruptions to these markers or the presence of abnormal proteins—such as those found in cancer, pathogens, microbes and foreign substances— cause the activation of the immune system.

Macrophages and monocyte-derived macrophages share the ability to phagocytose and secrete cytokines. The key difference between monocytes and macrophages is developmental: some macrophages are derived from monocytes, while some are of embryonic origin. Monocytes can differentiate into macrophages or dendritic cells once out of the spleen and in the target tissue.

Macrophages are found in most tissue systems, which stands to reason given their critical function in cancer and pathogen immune vigilance. Macrophages can adopt multiple phenotypes, the most common of which are the pro-inflammatory M1 phenotype and the anti-inflammatory M2 phenotype. These distinct subtypes serve different roles in the immune system. The ratio of the two populations can give important information about the conditions of a tissue microenvironment. M2 macrophages mediate tissue re-modelling and repair, angiogenesis, and growth once an infection or cancer is cleared. These types are implicated in the pro tumour microenvironment. M1 macrophages are pro-inflammatory and capable of facilitating Th1 responses and local tissue damage. Polarisation is generally though to be dynamic, and on a spectrum rather than binary.

Tissue-resident macrophages are among the first lines of defence in the immune system. They are known by many names across tissue systems, and key subspecies and markers are detailed below.