e of red blood cells), and are polymorphonuclear (meaning their nuclei have a variable shape with several lobes; neutrophils are also called PMNs or polys, short for polymorphonuclear neutrophilic leukocytes). The cytoplasm contains many lysosomal granules (vesicles containing stored lysosomal enzymes) whose specific staining properties give these cells their name. The granules are neutrophilic, meaning they do not show a special affinity for either acidic or basic stains, but are stained mildly by both. This is in contrast to the specific granules of eosinophils, which stain red with eosin, and basophils, which stain with basic stains. Severe inflammation may increase the numbers of neutrophils in blood, resulting in neutrophilia (blood smear image from WebPath). To view an electron micrograph, see WebPath.
Neutrophils are only occasionally seen in tissue sections outside blood (except, of course, in inflamed tissue). Here they may be most easily recognized by their lobed nuclei. One neutrophil nucleus might be mistaken for a cluster of very small nuclei, but each of the lobes is much smaller than any whole nucleus--only two or three Ám across, much smaller than the nuclei of lymphocytes which are among the smallest cells.
[More on blood cells.]
Lymphocytes accumulate somewhat later during the inflammatory process. Their presence in large numbers indicates the continuing presence of antigen and thus may suggest an established infection. Lymphocytes produce the multitude of diverse antibody molecules (one specific type of antibody per lymphocyte) which provide the mechanism for chemical recognition of foreign materials (distinguishing between self and non-self) and so for mediating and regulating immune responses.
Lymphocytes travel in the blood, but they routinely leave capillaries and wander through connective tissue. Therefore, lymphocytes may be normally encountered at any time in any location. They even enter epithelial tissue, crawling between the epithelial cells. They reenter circulation via lymphatic system channels (hence their name). Lymph channels drain into lymph nodes, where dense aggregations of lymphocytes form lymph nodules. Each lymph nodule has a "germinal center", where activated lymphocytes proliferate. Lymph nodules with proliferating lymphocytes also characterize the tonsils and the appendix and may be encountered in other sites as well.
[Recent research suggests that some types of lymphocytes are compartmentalized to particular tissues or body regions.]
Lymphocytes are small cells, 7-9 Ám in diameter in blood smears, and are the second most common white blood cell type (about 30% of the WBCs). They have a round heterochromatic (deeply staining) nucleus surrounded by a relatively thin rim of cytoplasm. Lymphocytes are most easily recognized in histological sections as small "naked" nuclei (the cytoplasm is usually inconspicuous) which occur here and there in most tissues and especially commonly near mucous membranes. Lymphocytes are found densely packed in lymphoid tissue--spleen and lymph nodes. To view an electron micrograph, see WebPath.
Plasma cells are lymphocytes which are specialized for mass production and secretion of circulating antibodies. Plasma cells have more extensive cytoplasm filled with rough endoplasmic reticulum (for synthesizing protein, specifically antibody molecules). This cytoplasm is distinctly basophilic, a consequence of the large numbers of ribosomes associated with the rER, and typically forms a lopsided bulge on one side of the nucleus. The heterochromatin of plasma cells is typically clumped in a characteristic "spoke-wheel" arrangement which also aids plasma cell recognition. To view an electron micrograph, see WebPath.
[More on blood cells.]
Monocytes are phagocytic cells which circulate in the blood. An equivalent cell type, called the macrophage, is a resident cell in connective tissue.
Monocytes/macrophages engulf and digest foreign microorganisms, dead or worn-out cells, and other tissue debris. They interact closely with lymphocytes to recognize and destroy fo