Myeloid cells represent a diverse group of immune cells crucial for the body’s defense. They are widely distributed throughout the body, playing various roles in recognizing and responding to threats. Their presence is a constant surveillance mechanism, ready to act against foreign invaders and maintain bodily equilibrium.
Defining Myeloid Cells
Myeloid cells are a core component of the innate immune system, which provides the body’s immediate, non-specific defense against pathogens. These cells are characterized by their rapid response time, often being among the first responders to infection or injury. Unlike the adaptive immune system, which develops tailored responses and immunological memory, myeloid cells target threats in a generalized manner. They recognize common patterns associated with pathogens and damaged cells, initiating a swift defensive action.
Myeloid cells contrast with lymphoid cells, such as T cells and B cells, which are primarily involved in the adaptive immune system. While both originate from hematopoietic stem cells, their developmental pathways and primary functions diverge significantly. Myeloid cells focus on immediate, broad-spectrum defense, whereas lymphoid cells specialize in highly specific recognition and memory formation. This division of labor ensures a comprehensive and layered approach to immune protection.
Where Myeloid Cells Come From
All blood cells, including the diverse array of myeloid cells, originate from hematopoietic stem cells (HSCs) found within the bone marrow. These multipotent stem cells possess the remarkable ability to differentiate into all types of blood and immune cells. The journey from a stem cell to a mature myeloid cell involves a series of differentiation steps.
HSCs initially give rise to more specialized progenitor cells. One such lineage is the common myeloid progenitor (CMP). This common myeloid progenitor acts as the precursor from which all various myeloid cell types will subsequently develop. The process of myeloid cell development, termed myelopoiesis, ensures a continuous supply of these cells to maintain immune surveillance and response throughout the body.
Key Types of Myeloid Cells
Neutrophils
Neutrophils are the most abundant type of granulocyte, making up about 40% to 70% of all white blood cells in humans. They are highly motile and are usually the first immune cells to arrive at a site of infection or injury, attracted by chemical signals. Neutrophils are actively phagocytic, meaning they engulf and digest bacteria, fungi, and other microscopic particles. Their cytoplasm contains numerous granules that stain a neutral pink, distinguishing them from other granulocytes.
Macrophages
Macrophages are specialized, long-lived phagocytic cells that are widely distributed throughout nearly all tissues of the body. They originate from monocytes, a type of white blood cell that circulates in the blood before migrating into tissues and differentiating into macrophages. Macrophages detect, engulf, and destroy bacteria, cellular debris, and other harmful organisms through phagocytosis. They also present antigens to T cells, thereby initiating adaptive immune responses, and release signaling molecules called cytokines that activate other immune cells. Macrophages contribute to tissue repair by clearing dead cells and promoting healing after inflammation.
Dendritic Cells
Dendritic cells (DCs) are antigen-presenting cells named for their distinctive, branching projections that resemble tree dendrites. They act as messengers between the innate and adaptive immune systems, playing a role in initiating and regulating immune responses. DCs are found in tissues that interact with the external environment, such as the skin, and also circulate in an immature state in the blood. Their primary function involves capturing and processing antigens from their surroundings, breaking them down into smaller fragments. These processed antigens are then presented on the cell surface to T cells, activating a precise defense against pathogens.
Eosinophils
Eosinophils are a type of white blood cell characterized by their large granules that stain bright red with acidic dyes like eosin. These cells are larger than most other white blood cells and typically have a two-lobed nucleus. Eosinophils are primarily involved in defending the body against parasitic infections and mediating allergic reactions. They release toxic proteins from their granules that can destroy parasites and contribute to inflammatory responses. While they are phagocytic, they are generally considered less efficient at killing intracellular bacteria than neutrophils.
Basophils
Basophils are the least numerous type of granulocyte, accounting for less than 1% of all white blood cells in humans. Their large granules stain purple-black, often obscuring their underlying double-lobed nucleus. Basophils are involved in allergic and inflammatory responses, synthesizing and storing histamine, a natural modulator of inflammation. When antibodies, specifically immunoglobulin E (IgE), bind to receptors on basophils, the cells release their stores of inflammatory chemicals, including histamine, serotonin, and leukotrienes. Basophils also contain heparin, a naturally occurring blood-thinning substance, which helps prevent blood clotting and promotes blood flow to damaged areas.
Mast Cells
Mast cells are immune cells found predominantly in connective tissues throughout the body, particularly at interfaces with the external environment like the skin, lung airways, and digestive tract. They are produced in the bone marrow and mature in tissues. Mast cells act as an alert system, recognizing harmful invaders such as parasites and toxins. Upon activation, often by IgE antibodies binding to their surface receptors, mast cells rapidly release a variety of mediators from their granules, including histamine, proteases, prostanoids, and leukotrienes. These mediators contribute to allergic reactions, inflammation, and the expulsion of parasites.
Myeloid Cells and Immune Protection
Myeloid cells collectively contribute significantly to innate immunity, acting as the body’s first line of defense against a wide array of threats. Their diverse functions are orchestrated to recognize, neutralize, and clear pathogens, as well as to initiate and regulate subsequent immune responses. This coordinated effort is fundamental for maintaining overall health and preventing widespread infection.
One primary mechanism is phagocytosis, where myeloid cells engulf and digest foreign particles, bacteria, fungi, and cellular debris. Inflammation is another key protective function, as myeloid cells release mediators like histamine in response to allergens or pathogens. This causes blood vessels to dilate and become more permeable, increasing blood flow and allowing immune cells to reach infection or injury sites. Antigen presentation, performed by certain myeloid cells, bridges the innate and adaptive immune systems by processing foreign material into antigens and presenting them to T cells, activating a more specific immune response. Finally, myeloid cells also play a role in tissue repair and remodeling after injury or infection, clearing away damaged cells and debris and contributing to regeneration.