Macrophage

Lore & Background

Macrophages take various forms throughout the body, including histiocytes, Kupffer cells, alveolar macrophages, and microglia, all part of the mononuclear phagocyte system. They can be classified as classically activated (M1) macrophages, which encourage inflammation, and alternatively activated (M2) macrophages, which decrease inflammation and encourage tissue repair. This dichotomy has been recently questioned as further complexity has been discovered.

Reader's Guide

Macrophages are significant because they serve as the first line of defense against infection and injury through phagocytosis, and they also bridge innate and adaptive immunity by recruiting other immune cells such as lymphocytes and presenting antigens to T cells. Their dysfunction can cause severe diseases such as chronic granulomatous disease, which results in frequent infections. Beyond their role in inflammation, macrophages also play an important anti-inflammatory role and can decrease immune reactions through the release of cytokines. They are widely thought of as highly plastic and fluid cells, with a fluctuating phenotype. Their ability to metabolize arginine differently—M1 macrophages to nitric oxide and M2 macrophages to ornithine—reflects their functional dichotomy. Macrophages survive longer in the body than neutrophils, up to several months, and are involved in tissue repair, electrical conduction in the heart, and maintaining immune privilege in organs such as the testis.

Did You Know?

• Human macrophages are about 21 micrometres in diameter.
• M1 macrophages metabolize arginine to nitric oxide, while M2 macrophages metabolize arginine to ornithine.
• Macrophages can survive up to several months in the body.

The Cell That Patrols Every Corner

Élie Metchnikoff observed these cells engulfing and digesting foreign invaders, and the process he described—phagocytosis—remains the defining activity of the macrophage to this day. Roughly 21 micrometres across, these cells are produced when monocytes migrate into tissues and differentiate, and they patrol essentially every tissue in the body using slow, amoeboid movement. Their target is anything that lacks the surface proteins characteristic of healthy body cells: microbes, cancer cells, dead cellular debris, and other foreign material. This strategy of self-protection stands in contrast to the approach used by Natural Killer cells. Although they wear many local names—histiocytes in connective tissue, Kupffer cells in the liver, alveolar macrophages in the lungs, microglia in the brain—they all belong to a single family called the mononuclear phagocyte system, and together they form a first-line defense against infection and injury.

The Second Wave: Cleaning Up After Neutrophils

When tissue is wounded, the immune response unfolds in distinct waves. The first responders are neutrophils, short-lived cells that rush to the site, carry out their destructive work, and then die. Their corpses, along with the extracellular traps they leave behind, do not simply linger. Within roughly two days of the initial injury, macrophages arrive at the wound site to perform a critical cleanup. They engulf and digest the spent neutrophils, a process that is central to resolving chronic inflammation. Beyond this scavenging role, macrophages are professional phagocytes: when they capture a pathogen, the invader is sealed inside a phagosome that fuses with a lysosome. The resulting phagolysosome bathes the microbe in digestive enzymes and toxic peroxides, breaking it down. Not every pathogen succumbs, however; organisms such as Mycobacterium tuberculosis have evolved resistance to this intracellular digestion. Fixed macrophages stationed in the lungs, liver, neural tissue, bone, spleen, and connective tissue handle much of the routine clearance of dying cells—a task called efferocytosis—and can recruit additional macrophages when the workload demands it.

Two Faces, One Fluid Cell

Macrophages are not simply agents of inflammation; they also actively dampen immune reactions and promote tissue repair. This dual capacity is captured in the M1/M2 framework. Classically activated M1 macrophages drive inflammation and possess the unique metabolic ability to convert the amino acid arginine into nitric oxide, a molecule with potent antimicrobial properties. In contrast, alternatively activated M2 macrophages favor wound healing and tissue repair, and they uniquely channel arginine toward ornithine, a building block associated with regeneration. A third category, regulatory macrophages, has also been described. Yet researchers increasingly recognize that this tidy two-type model oversimplifies reality. Further complexity has emerged, and macrophages are now widely regarded as highly plastic, fluid cells whose phenotype fluctuates in response to their local environment. Beyond their inflammatory and anti-inflammatory roles, macrophages serve as antigen-presenting cells to T lymphocytes, thereby bridging innate and adaptive immunity. When their function breaks down, the consequences are severe: conditions such as chronic granulomatous disease leave patients vulnerable to repeated, life-threatening infections.

Citizens of Every Organ

Although all macrophages share a common lineage, their behavior is profoundly shaped by the organ they inhabit. In the testis, resident macrophages secrete 25-hydroxycholesterol, an oxysterol that neighboring Leydig cells can convert into testosterone; they also help maintain an immune-privileged environment and can mediate infertility when testicular inflammation occurs. In the heart, macrophages participate in electrical conduction by forming gap junctions with cardiac myocytes. Kupffer cells in the liver pose particular research challenges: in humans they can only be studied through biopsies or autopsies, and in laboratory mice, purification yields only about five million cells per animal. The developmental origins of these tissue residents vary as well. Some macrophages in healthy adult tissues were established before birth and are maintained independently of the monocyte pool, while those that accumulate at diseased sites typically derive from circulating monocytes entering through the vascular endothelium via leukocyte extravasation. At sites like the testis, local proliferation also contributes to the macrophage population. Unlike short-lived neutrophils, these long-lived cells can persist for months, identified in the laboratory by surface markers including CD14, CD68, CD64, and CD11b.