Myeloid cells are a diverse group of cells that play an important role in the body’s defense mechanisms. These cells originate from hematopoietic stem cells, which are multipotent cells found in the bone marrow. From these stem cells, myeloid cells differentiate and mature into various specialized types, forming a part of both the innate and adaptive immune systems. They protect the body from infections and maintain tissue health.
The Myeloid Family
The myeloid family includes specialized cells, tracing their lineage back to common myeloid progenitor cells in the bone marrow. These progenitor cells differentiate into distinct mature blood cells with unique characteristics and locations. This ensures a continuous supply of myeloid cells.
Neutrophils are the most abundant type of granulocyte and act as first responders at sites of inflammation and infection. They are found in the blood and recruited to damaged tissues. Eosinophils, a granulocyte, defend against parasitic infections and contribute to allergic reactions, found in tissues like the respiratory and gastrointestinal tracts. Basophils, a granulocyte, contribute to inflammation by releasing substances like histamine and heparin, found in blood and tissues during allergic responses.
Macrophages develop from monocytes, circulating in the blood before migrating to tissues. In tissues, monocytes differentiate into macrophages, distributed throughout the body, with specialized forms in organs like the liver (Kupffer cells) and brain (microglia). Dendritic cells, originating from monocytes or precursors, are in tissues exposed to the external environment, such as skin (Langerhans cells) and mucous membranes, acting as sentinels. Mast cells, sharing characteristics with basophils, mature in tissues and are abundant in connective tissues near blood vessels and nerves, playing a role in allergic responses and wound healing.
Guardians of the Body
Myeloid cells perform diverse functions central to the body’s protective mechanisms. A primary role involves their contribution to innate immunity, the body’s immediate, non-specific defense system. Neutrophils, for example, are highly efficient phagocytes, engulfing and destroying invading pathogens like bacteria and fungi at infection sites. Macrophages also employ phagocytosis, clearing cellular debris, dead cells, and pathogens, contributing significantly to tissue cleanup and immune surveillance.
Beyond direct pathogen clearance, myeloid cells initiate and regulate inflammatory responses. Basophils and mast cells release chemical mediators like histamine, which increase blood flow and recruit other immune cells to the site of injury or infection. This localized inflammation helps contain the threat and promote healing. Eosinophils, while known for fighting parasites, also release specific proteins and enzymes that can damage pathogens or contribute to tissue remodeling.
Myeloid cells also form a bridge to adaptive immunity, the body’s more specific and long-lasting defense system. Dendritic cells are specialized antigen-presenting cells that capture antigens from pathogens or abnormal cells in tissues. They then migrate to lymph nodes, where they present these antigens to T lymphocytes, initiating specific adaptive immune responses. Macrophages can also act as antigen-presenting cells, further linking the innate and adaptive arms of the immune system. This interaction ensures a comprehensive and coordinated defense against a wide range of threats.
Myeloid Cells in Health and Disease
Myeloid cells are important for maintaining health, but their dysregulation can contribute to various disease states. In chronic inflammatory conditions, such as rheumatoid arthritis or inflammatory bowel disease, persistently activated macrophages can release pro-inflammatory cytokines, leading to ongoing tissue damage. This sustained inflammation can perpetuate the disease cycle rather than resolve it. Similarly, in autoimmune disorders, myeloid cells might mistakenly target the body’s own tissues, contributing to the pathology.
Myeloid cells also play a complex and dual role in cancer. Tumor-associated macrophages (TAMs) are frequently found in the tumor microenvironment and can promote tumor growth, angiogenesis (new blood vessel formation), and metastasis, rather than suppressing the cancer. Myeloid-derived suppressor cells (MDSCs) are another population of myeloid cells that can accumulate in cancer patients and suppress anti-tumor immune responses, allowing cancer cells to evade detection and destruction.
Dysregulation in the development or function of myeloid cells can lead to specific blood disorders. Myeloid malignancies, such as acute myeloid leukemia (AML) and chronic myeloid leukemia (CML), arise from uncontrolled proliferation and abnormal differentiation of myeloid progenitor cells in the bone marrow. In AML, immature myeloid cells (blasts) accumulate, impairing the production of normal blood cells. CML involves an overproduction of mature and immature myeloid cells, particularly granulocytes. Understanding these cellular alterations is important for diagnosing and developing targeted therapies for such conditions.