Definition and function:

macrophages (ancient Greek 'macros' = large; 'phagein' = eating) belong to the white blood cells (leucocytes) as so-called phagocytes. They are an important component of the innate immune response and responsible for the destruction of invading pathogens, such as bacteria, viruses or toxins.
The formation of macrophages begins in the bone marrow. There originate from stem cells the progenitor cells of the macrophages, the monocytes. In the bloodstream, the monocytes only differentiate into macrophages through contact with cytokines (e.g., interferons and interleukins). From then on, the lifespan is between 30 and 90 days.
The most important function of the phagocytes is the detection and destruction of pathogens. Based on the proteins on the cell walls of the pathogens, macrophages identify the foreign cells and begin immediately with the phagocytosis:
1. The foreign cell is completely enclosed by a macrophage.
2. The trapped pathogen is now in the phagosome, a cell organelle of macrophages.
3. Several lysosomes merge with the phagosome.
4. The digestive enzymes from the lysosomes dissolve the pathogen.
5. During phagocytosis, the macrophage releases messengers (chemokines) to attract more macrophages.
6. Other organic matter of the decomposed pathogen releases the macrophages to the outside again.
With a size of 25-50 microns, the macrophages are significantly larger than the pathogens to be controlled (the majority of the bacteria is between 0.5 and 5 microns long). Otherwise, the phagocytes would also be unable to phagocytose pathogens, since the pathogen must be completely enclosed because the digestive enzymes must not enter the extracellular space. In this case, it would otherwise come into contact with the body's own proteins.
In addition, scavenger cells fulfill a wealth of other tasks in the service of the immune system:
antigen presentation to identify endogenous / foreign cells
production of lysozyme
Elicitation of inflammation for faster healing
Destruction of endogenous, already dead, cells