tem 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 foreign substances.
Resident macrophages normally remain at rest (rather than circulating in and out of tissues like the lymphocytes). But the normal number of fixed macrophages is supplemented during inflammation by the influx of many monocytes from the blood. When faced with a target too big for one cell, a splinter for example, several macrophages may fuse together for a single huge multinucleate mass called a "foreign body giant cell."
Monocytes are the largest of the leukocytes, and constitute about 5% of the white blood cell population. Their nuclei are typically indented, with a kidney-bean shape, in blood smears. Tissue macrophages are diverse in appearance and not easily distinguished from the more common fibroblasts. Macrophages are generally larger, and may contain brown pigment granules which represent indigestible residue in tertiary lysosomes. In electron micrographs macrophages are generally recognized by the presence of numerous lysosomes of various sizes, including large heterophagic vesicles.
[More on blood cells.]
For more about the immune system, including outside links, see CRR Lymphatic System.
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EXAMPLES of inflammation
Microscopically, inflammation shows up most clearly as increased numbers of white blood cells (i.e., the inflammatory infiltrate which results from emigration of leukocytes).
Images of inflammatory infiltrate in skin.
The WebPath website offers a number of illustrative examples of inflammation, including:
- cholecycitis (inflammation of the gall bladder)
- endometritis (inflammation of the lining of the uterus)
- cervicitis (inflammation of the cervix)
- rheumatoid arthritis (inflammation of joints)
- pneumonia (inflammatory exudate in lung); abscessing pneumonia
- influenza (interstitial inflammation of lung)
- more from WebPath
Also see specimens at Virtual Slidebox of Histopathology, University of Iowa Department of Pathology.
Online slides of cell injury, inflammation, and repair
Note that appreciating inflamed tissue samples, such as those listed above, calls fo