Myeloid cells are involved in innate and adaptive immunity. They include granulocytes (neutrophils, eosinophils, basophils), monocytes and macrophages, dendritic cells and mast cells.
Granulocytes. Granulocytes have densely staining cytoplasmic granules and have multilobcd nuclei (hence also known as polymorphonuclear leucocytes or PMNs).
Neutrophils are short-lived, mobile phagocytes, which circulate in the bloodstream until recruited to sites of inflammation. They are the most numerous cells of the innate immune response, but also interact with antibodies playing an effector role in adaptive responses. Eosinophils play a role in defence against parasitic infections and are also recruited to sites of allergic inflammation. The function of basophils is uncertain.
Monocytes and macrophages. Monocytes circulate in the blood, migrate into tissues and differentiate into macrophages. Macrophages are part of the mononuclear phagocyte system (previously known as the reticuloendothelial system) and are distributed widely in body tissues. Macrophages play an important role in innate immunity and also present antigen to T cells.
Dendritic cells. Dendritic cells are professional APCs, which migrate from blood into tissues. On encountering a pathogen, dendritic cells ingest antigen by phagocytosis, mature and migrate to lymph nodes where they present antigen to T cells and activate T cells.
Mast cells. Mast cells are found in connective tissue and close to mucosal surfaces. They trigger local inflammatory responses to antigen by rapidly releasing inflammatory mediators including histamine. Mast cells play a pivotal role in allergic responses.
