MonozytenMakrophagenSystems

The monocyte-macrophage system, also known as the mononuclear phagocyte system, comprises circulating monocytes and tissue-resident macrophages that originate from these precursors. Monocytes are produced in the bone marrow and enter the bloodstream, where they patrol for signs of infection or tissue damage before migrating into tissues to differentiate.

In tissues, monocytes differentiate into macrophages or dendritic cells in response to local signals. Macrophages are highly diverse and occupy many organs, including the liver (Kupffer cells), brain (microglia), lungs (alveolar macrophages), and other tissues. Some tissue macrophages originate during embryonic development and persist independently of circulating monocytes, contributing to foundational tissue homeostasis.

Functions of monocytes and macrophages include phagocytosis of pathogens, dead cells, and debris; production of cytokines and chemokines that shape inflammation; and antigen presentation to T cells via MHC class II molecules. They play key roles in frontline defense, wound healing, remodeling of tissues, and resolution of inflammation.

Monocyte and macrophage populations show diversity. Monocytes can be subdivided into subsets with different surface markers and functions, while macrophages exhibit tissue-specific characteristics. A polarization framework describes macrophage activation states (often termed M1 and M2) that influence inflammatory responses and tissue repair, though the exact in vivo roles are context-dependent.

Clinical relevance arises from their involvement in infections, chronic inflammatory diseases, atherosclerosis, and fibrosis. Because of their central role in immunity and tissue dynamics, monocytes and macrophages are common targets of diagnostic and therapeutic strategies in various diseases.