Myeloid cells are a diverse group of white blood cells that form a fundamental part of the body’s immune system. They act as a first line of defense, recognizing and responding to invading pathogens and foreign substances. These cells are essential for protecting the body from infections and maintaining overall health.
Origin of Myeloid Cells
Myeloid cells originate from hematopoietic stem cells in the bone marrow. These stem cells can develop into all blood cell types through hematopoiesis, the process of blood cell formation. A common myeloid progenitor cell then gives rise to the various myeloid cell types.
Diverse Myeloid Cell Types
The myeloid lineage encompasses several distinct cell types, each with unique characteristics. Neutrophils are the most abundant white blood cells, characterized by a multi-lobed nucleus and granules that stain neutrally. Macrophages develop from monocytes, which circulate in the blood before migrating into tissues, where they mature into large phagocytic cells. Dendritic cells, named for their tree-like projections, are found in tissues that interact with the external environment, such as the skin and mucous membranes.
Mast cells are tissue-resident immune cells containing numerous granules rich in histamine and heparin, found near blood vessels and nerves. Eosinophils are identifiable by their two-lobed nucleus and large granules that stain red or pink with acidic dyes like eosin. Basophils are the least common granulocyte, possessing large granules that often obscure their double-lobed nucleus and stain dark purple.
Fundamental Roles in the Immune System
Myeloid cells perform essential functions in innate immunity. Neutrophils are the first responders to sites of infection, engulfing and destroying bacteria through phagocytosis. Macrophages also engage in phagocytosis, clearing pathogens, dead cells, and debris. They can also present antigens to T cells, activating adaptive immune responses. Dendritic cells specialize in capturing and processing antigens, linking the innate and adaptive immune systems by presenting these antigens to T cells.
Mast cells and basophils contribute to inflammatory and allergic reactions. Upon activation, mast cells release chemical mediators like histamine from their granules, increasing blood vessel permeability and causing inflammation. Basophils, while less common, also release histamine and heparin, playing a role in allergic responses and anti-parasitic immunity. Eosinophils are involved in defending against parasitic infections and modulating allergic reactions by releasing toxic proteins from their granules.
Myeloid Cells and Body Balance
Myeloid cells maintain the body’s overall health and balance. Beyond fighting infections, these cells contribute to tissue repair and wound healing. Macrophages, for instance, clear damaged tissue and secrete growth factors that aid regeneration. Coordinated myeloid cell actions help resolve inflammation, restoring tissue homeostasis.
Dysregulation in myeloid cell activity can lead to chronic inflammatory conditions. For example, excessive eosinophil activity can cause tissue damage in allergic disorders. Understanding the balance of myeloid cell functions is important for comprehending protective immunity and immune-related health challenges.