Myeloid cells are a group of specialized white blood cells that form a major part of the body’s defense system. These cells are diverse, with unique functions contributing to the body’s protective mechanisms. Their actions are fundamental to identifying and eliminating harmful invaders and managing tissue processes.
Understanding Myeloid Cells
Myeloid cells are a type of white blood cell, also known as leukocytes, that originate from hematopoietic stem cells (HSCs) found in the bone marrow. This is the myeloid lineage. These stem cells are multipotent, differentiating into various blood cells, including red blood cells, platelets, and diverse myeloid cells.
The fundamental role of myeloid cells is within the innate immune system, the body’s immediate, non-specific first line of defense against infection and injury. This distinguishes them from lymphoid cells, such as B and T lymphocytes, which are part of the adaptive immune system. Myeloid cells are continuously produced in the bone marrow through a process called myelopoiesis.
The Many Faces of Myeloid Cells
Neutrophils
Neutrophils are often the first responders to infection or injury, rapidly migrating to affected sites. These highly abundant white blood cells primarily function through phagocytosis, engulfing and digesting harmful microorganisms like bacteria and fungi. They also release antimicrobial proteins and form neutrophil extracellular traps (NETs) to trap and kill pathogens.
Macrophages
Macrophages are immune cells found in nearly all tissues, where they patrol for pathogens, cellular debris, and foreign substances. They are phagocytes, capable of engulfing and digesting a wide range of targets, including cancer cells and damaged cells. Macrophages also present antigens to T cells, helping to initiate adaptive immune responses, and contribute to tissue repair.
Dendritic Cells
Dendritic cells are specialized antigen-presenting cells that bridge the innate and adaptive immune systems. They capture and process antigens from pathogens or abnormal cells in peripheral tissues, then migrate to lymph nodes to present these antigens to T cells. This presentation activates T cells, guiding them to mount specific immune responses.
Monocytes
Monocytes circulate in the blood and serve as precursors to macrophages and dendritic cells. When an infection or injury occurs, monocytes migrate from the bloodstream into tissues, where they differentiate into these more specialized immune cells. Monocytes also directly engage in phagocytosis and produce cytokines to alert other immune cells.
Eosinophils
Eosinophils are granulocytes involved in defending against parasitic infections and contributing to allergic reactions. They contain granules filled with enzymes and proteins that can be released to kill parasites and modulate inflammatory responses.
Basophils and Mast Cells
Basophils and mast cells are closely related granulocytes involved in allergic reactions and inflammatory responses. Basophils circulate in the blood, while mast cells reside in tissues. Both cell types possess high-affinity receptors for IgE antibodies and, upon activation by allergens, release mediators such as histamine, which contributes to symptoms.
Myeloid Cells in Health
Myeloid cells are instrumental in controlling infections. Neutrophils, for example, rapidly arrive at infection sites and engulf bacteria, fungi, and other microorganisms through phagocytosis. Macrophages also contribute by consuming pathogens and infected cells, while dendritic cells capture microbial antigens to initiate broader immune responses.
These cells play a role in initiating and resolving inflammation, a protective mechanism against injury and infection. Initially, myeloid cells release pro-inflammatory mediators that recruit more immune cells to the site of damage. As the threat is cleared, macrophages transition to an anti-inflammatory state, clearing apoptotic cells and promoting tissue balance.
Macrophages are involved in tissue repair and maintaining homeostasis, which is the body’s ability to maintain stable internal conditions. They clear cellular debris and dead cells, which is essential for wound healing. Myeloid cells also produce growth factors and signaling molecules that support the regeneration of damaged tissues.
Dendritic cells and macrophages serve as a bridge between the innate and adaptive immune systems. Dendritic cells present processed antigens to T cells, guiding the adaptive response to be specific and effective against particular threats. This interaction ensures that the immune system learns from past encounters and mounts a more targeted response in the future.
Myeloid Cells in Disease
Myeloid cells can contribute to chronic inflammation and autoimmune disorders when their functions become dysregulated. In conditions like rheumatoid arthritis, inflammatory bowel disease, or asthma, persistent activation of macrophages, mast cells, and eosinophils can lead to ongoing tissue damage. This sustained activity may result from a failure to resolve acute inflammation or a misguided immune response against the body’s own tissues.
In cancer, myeloid cells often play a complex role within the tumor microenvironment. Tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) can promote tumor growth by fostering angiogenesis, which is the formation of new blood vessels that feed the tumor. These cells also suppress anti-tumor immunity, allowing cancer cells to evade detection and destruction by other immune cells.
Myeloid leukemias are a group of cancers characterized by the uncontrolled growth of immature myeloid cells. Acute myeloid leukemia (AML), for instance, involves the rapid accumulation of abnormal myeloblasts in the bone marrow and blood. These immature cells do not function properly and crowd out healthy blood cell production, leading to symptoms such as increased susceptibility to infections, anemia, and easy bruising.