Macrophages are specialized cells involved in detection, phagocytosis and destruction of bacteria and other harmful organisms. In addition, they can also present antigens to T cells and initiate inflammation by releasing molecules (such as cytokines) that activate other cells.
Mononuclear phagocytic system includes circulating cells called monocytes and tissue resident cells called macrophages. Monocyte is considered a leukocyte in transit through the blood, which becomes a macrophage when fixed in a tissue. Macrophages, widely distributed in organs and connective tissue plays central roles in innate and adaptive immunity.
Differentiation of a monocyte into a tissue macrophage involves number of changes which are as follows:
- Enlargement of cell (5–10 folds).
- Production of higher levels of hydrolytic enzymes.
- Increase in number and complexity of intracellular organelles.
- Begins to secrete a variety of soluble factors.
- Increased phagocytic ability.
Monocytes and macrophages as well as granulocytes are able to ingest particulate matter (microorganisms, cells, inert particles) and are said to have phagocytic functions. The phagocytic activity is greater in macrophages particularly after activation by soluble mediators released during immune responses than in monocytes.
To participate in immune response, macrophages need to be stimulated and reach an activated state. Macrophages can be activated by various components of bacterial cell wall, cytokines, mediators of inflammatory response etc. Gamma interferon produced by helper T cells is a potent activator of macrophages and is secreted by various cells in response to appropriate stimuli. Bacterial lipopolysaccharides (endotoxin), bacterial peptidoglycan, and bacterial DNA can also activate macrophages.
Activated macrophages are more potent than normal macrophages in many ways, such as having greater phagocytic ability and increased ability to kill ingested microbes. They are better APCs (Antigen Presenting Cells) and they activate T-cell response in a more effective manner. By secreting various cytotoxic proteins, they help in eliminating a broad range of pathogens, including virus-infected cells, tumor cells, and intracellular bacteria.
Macrophage-like cells serve different functions in different tissues and are named according to their tissue location. Some examples includes:
- Mesangial cells in kidneys.
- Alveolar macrophages in lungs.
- Microglial cells in brain.
- Osteoclasts in bone.
- Kupffer cells in liver.
- Histiocytes in connective tissues
In addition to ingesting microbes, macrophages also ingest dead host cells, including cells that die in tissues because of trauma or interrupted blood supply and neutrophils that accumulate at sites of infection.