Macrophages (Greek: big eaters, from Greek μακρος (makros) = large, φαγειν (phagein) = to eat) are a type of white blood cell that engulfs and digests cellular debris, foreign substances, microbes, cancer cells, and anything else that does not have the protein markers specific to healthy body cells on its surface, in a process called phagocytosis.
Macrophages are large phagocytes and are found in all tissues, where they patrol for potential pathogens and cellular waste to dispose of.
Macrophages have a number of tissue-specific variants throughout the body (e.g., microglia, histiocytes, osteoclasts, Kupffer cells, and others), but all are part of the mononuclear phagocyte system and perform essentially the same housekeeping role within the body.
Macrophages develop in the bone marrow as monocytes, which circulate in the bloodstream until they settle in the tissues and become one of the many tissue-specific types of macrophage.
Besides phagocytosis, macrophages play a critical role in nonspecific defense (innate immunity) and also help initiate specific defense mechanisms (adaptive immunity) by recruiting other immune cells, such as lymphocytes, to help them fight pathogens. In humans, dysfunctional macrophages cause severe diseases, such as chronic granulomatous disease, that result in frequent infections; aging causes macrophages to become increasingly poor at cellular housekeeping and fighting pathogens.
A number of research efforts have focused on trying to return aged, dysfunctional macrophages to more youthful levels of function by modulation of the immune system (in particular microglia – macrophages) using pro-youthful signalling.
One such effort in 2016 used mesenchymal stem cells sent to the area of damage and resulted in a reduction of plaques in Alzheimer’s mice as these stem cells reduce inflammation and encourage pro-youthful function via intracellular signalling.
Recently, researchers showed that delivering fresh, functionally young macrophages to the brain results in removal of the plaques (misfolded proteins) associated with Alzheimer’s disease.
This means that brain aging is not a one-way process, and suggests that we might be able to use our own rejuvenated macrophages to treat this horrific disease and other similar diseases, like heart disease and Parkinson’s, which involve plaque accumulation too.
Macrophages and boosting their efficiency represents a plausible pathway to treating age-related diseases, and the research world is currently investigating the potential of a number of therapies that do this.
 Naaldijk, Y., Jaeger, C., Fabian, C., Leovsky, C., Blüher, A., Rudolph, L., … & Stolzing, A. (2016). Effect of systemic transplantation of bone marrow‐derived mesenchymal stem cells on neuropathology markers in APP/PS1 Alzheimer’s mice. Neuropathology and applied neurobiology.
 Daria, A., Colombo, A., Llobera, G., Hampel, H., Willem, M., Liesz, A., … & Tahirovic, S. (2016). Young microglia restore amyloid plaque clearance of aged microglia. The EMBO Journal, e201694591.