Myeloid cells include highly phagocytic, motile neutrophils, monocytes, and macrophages that provide a first line of defense against most pathogens. The other myeloid cells, including eosinophils, basophils, and their tissue counterparts, mast cells, are involved in defense against parasites and in the genesis of allergic reactions. In contrast, lymphocytes regulate the action of other leukocytes (white blood cells) and generate specific immune responses that prevent chronic or recurrent infections.
Neutrophils: These are one of the major types of cells that are recruited to ingest, kill, and digest pathogens. Neutrophils are the most highly adherent, motile, phagocytic leukocytes and are the first cells recruited to acute inflammatory sites. Each of their functions is dependent upon special proteins, such as the adherence molecule CD11b/CD18, or biochemical pathways, such as the respiratory burst associated with cytochrome b558.
Eosinophils: Eosinophils defend against many types of parasites and participate in common hypersensitivity reactions via cytotoxicity. That cytotoxicity is mediated by large cytoplasmic granules, which contain the eosinophilic basic and cationic proteins.
Credit: CDC/ Dr. Candler Ballard
Basophils: These cells and their tissue counterparts, mast cells, produce cytokines that help defend against parasites and trigger allergic inflammation. These cells display high affinity surface membrane receptors for IgE antibodies and have many large cytoplasmic granules, which contain heparin and histamine. When cell-bound IgE antibodies are cross-linked by antigens, the cells degranulate and produce low-molecular weight vasoactive mediators such as histamine through which they exert their biological effects.
Monocytes/Macrophages: Monocytes and macrophages are involved in phagocytosis and intracellular killing of microorganisms. Macrophages process protein antigens and present peptides to T cells. These monocytes/macrophages are highly adherent, motile and phagocytic; they marshal and regulate other cells of the immune system, such as T lymphocytes; serve as antigen processing-presenting cells; and act as cytotoxic cells when armed with specific IgG antibodies.
Macrophages are differentiated monocytes, which are one of the principal cells found to reside for long periods in the reticuloendothelial system. Macrophages may also be recruited to inflammatory sites, and be further activated by exposure to certain cytokines to become more effective in their biologic functions.
- Medical Microbiology. 4th edition. Armond S. Goldman and Bellur S. Prabhakar