Myeloid cells are a broad category of white blood cells that serve as a fundamental part of the body’s immune system. They play a significant role in defending against infections and maintaining overall health. These cells are involved in both initial, non-specific innate immunity and contribute to more targeted, adaptive immune responses.
Origins and Development
Myeloid cells originate from hematopoietic stem cells (HSCs), multipotent stem cells in the bone marrow. This process, called hematopoiesis, involves HSCs differentiating into various specialized blood cells. HSCs can self-replicate and differentiate into different cell types, including those of the myeloid lineage.
The myeloid lineage begins with a common myeloid progenitor (CMP) cell, which further differentiates into various myeloid cell types. While most hematopoiesis occurs in the bone marrow during adulthood, some myeloid populations, such as tissue-resident macrophages, can develop directly from embryonic yolk sac progenitors.
Key Myeloid Cell Types and Their Roles
Neutrophils are the most abundant white blood cell, making up 50% to 70% of circulating leukocytes. They are often the first immune cells to arrive at sites of infection or injury, particularly against bacterial and fungal pathogens. Neutrophils employ several mechanisms to combat microbes, including phagocytosis (engulfing and digesting pathogens), degranulation (releasing antimicrobial substances), and forming neutrophil extracellular traps (NETs) that ensnare and kill microbes.
Macrophages are large, specialized white blood cells that originate from monocytes circulating in the blood. When monocytes migrate into tissues, they mature into macrophages, found in nearly all tissues throughout the body. Macrophages are phagocytes, engulfing and digesting cellular debris, worn-out cells, and pathogens like bacteria and viruses. They also play an important role as antigen-presenting cells (APCs), processing ingested microbes and displaying fragments of them (antigens) on their cell surface to activate T helper cells, thereby linking innate and adaptive immunity.
Dendritic cells (DCs) are effective antigen-presenting cells that act as messengers between the innate and adaptive immune systems. They are located in tissues exposed to the external environment, such as the skin, lungs, and gastrointestinal tract. Immature DCs capture antigens from pathogens and, upon maturation, migrate to lymph nodes where they present these antigens to T cells, initiating adaptive immune responses. Their ability to “cross-present” extracellular antigens on MHC class I molecules enables them to activate cytotoxic T cells.
Eosinophils are granulocytes involved in allergic reactions and defense against parasitic infections. They contain granules filled with toxic proteins, which they release to destroy parasites. Eosinophils also contribute to the inflammatory response in allergic conditions such as asthma and eczema.
Basophils are the least numerous white blood cells, but play a role in allergic responses and defense against parasites. They release histamine, which increases blood flow to damaged tissues and contributes to allergic symptoms like itching and a runny nose. Basophils also release heparin, an anticoagulant that helps prevent blood clots, ensuring proper blood flow for healing.
Mast cells are tissue-resident immune cells found throughout connective tissues, especially beneath the skin and near blood vessels. They store various chemical mediators, including histamine, in granules. Upon stimulation, mast cells release these mediators through degranulation. This release triggers local inflammatory responses characteristic of allergic reactions, such as swelling, smooth muscle contraction, and increased mucus production.
Myeloid Cells in Health and Disease
Myeloid cells importantly contribute to maintaining overall health by fighting infections and managing inflammatory responses. Their coordinated actions form a strong defense system that identifies and eliminates harmful invaders, clears cellular debris, and helps resolve inflammation. This process is important for tissue homeostasis and preventing widespread illness.
Dysregulation of myeloid cells can lead to health issues. In chronic inflammatory conditions, their prolonged or excessive activation can contribute to tissue damage. Myeloid cells are also implicated in autoimmune diseases, where the immune system mistakenly attacks the body’s own tissues. Certain types of cancers, such as myeloid leukemias, involve the uncontrolled growth and abnormal differentiation of myeloid cells. Examples include Acute Myeloid Leukemia (AML) and Chronic Myeloid Leukemia (CML), where normal cell development is disrupted, leading to dysfunctional cells.