Granulocytes contain numerous lysosomes and secretory vesicles (or granules) and are subdivided into three classes according to the morphology and staining properties of these organelles (Figure 22–27). The differences in staining reflect major differences of chemistry and function. Neutrophils (also called polymorphonuclear leukocytes because of their multilobed nucleus) are the most common type of granulocyte; they phagocytose and destroy microorganisms, especially bacteria, and thus have a key role in innate immunity to bacterial infection, as discussed in Chapter 24 (see Movie 16.1). Basophils secrete histamine (and, in some species, serotonin) to help mediate inflammatory reactions; they are closely related to mast cells, which reside in connective tissues but are also generated from the hematopoietic stem cells. Eosinophils help to destroy parasites and modulate allergic inflammatory responses.
Once they leave the bloodstream, monocytes (see Figure 22–27D) mature into macrophages, which, together with neutrophils, are the main “professional phagocytes” in the body. As discussed in Chapter 13, both types of phagocytic cells contain specialized lysosomes that fuse with newly formed phagocytic vesicles (phagosomes), exposing phagocytosed microorganisms to a barrage of enzymatically produced, highly reactive molecules of superoxide (O2–) and hypochlorite (ClO–, the active ingredient in bleach), as well as to attack by a concentrated mixture of lysosomal hydrolase enzymes that become activated in the phagosome. Macrophages, however, are much larger and longer-lived than neutrophils. They recognize and remove senescent, dead, and damaged cells in many tissues, and they are unique in being able to ingest large microorganisms such as protozoa.
Figure 22–27 White blood cells. (a–d) these electron micrographs show (a) a neutrophil, (b) a basophil, (C) an eosinophil, and (d) a monocyte. electron micrographs of lymphocytes are shown in Figure 24–14. each of the cell types shown here has a different function, which is reflected in the distinctive types of secretory granules and lysosomes it contains. there is only one nucleus per cell, but it has an irregular lobed shape, and in (a), (b), and (C) the connections between the lobes are out of the plane of section. (e) a light micrograph of a blood smear stained with the
Romanowsky stain, which colors the white blood cells strongly. (Courtesy 2 µm of dorothy bainton.)
Monocytes also give rise to dendritic cells. Like macrophages, dendritic cells are migratory cells that can ingest foreign substances and organisms, but they do not have as active an appetite for phagocytosis and instead have a crucial role as presenters of foreign antigens to lymphocytes to trigger an immune response. Dendritic cells in the epidermis (called Langerhans cells), for example, ingest foreign antigens and carry these trophies back from the skin to present to lymphocytes in lymph nodes.
There are two main classes of lymphocytes, both involved in immune responses: B lymphocytes make antibodies, while T lymphocytes kill virus-infected cells and regulate the activities of other white blood cells. In addition, there are lymphocyte-like cells called natural killer (NK) cells, which kill some types of tumor cells and virus-infected cells. The production of lymphocytes is a specialized topic discussed in detail in Chapter 24. Here we concentrate mainly on the development of the other blood cells, often referred to collectively as myeloid cells.
Table 22–1 summarizes the various types of blood cells and their functions.
the production of each type of blood Cell in the bone marrow is individually Controlled