Figure 910

Electron micrograph of a human eosinophil.

The nucleus is bilohed, but the connecting segment is not within the plane of section. The granules are of moderate size, compared with those of the basophil, and show a crystalline body (Cr) within the less electron-dense matrix of the granule. M, mitochondria. x26,000. (Courtesy of Dr. Dorothea Zucker-Franklin.) Inset Light microscopic image of an eosinophil from a blood smear. XI,800.

mast cells are present in blood but do not develop until they leave the circulation and lodge in tissue sites.

Lymphocytes

Lymphocytes are the main functional cells of the lymphatic or immune system

Lymphocytes are the most common agranulocytes and account for about 30% of the total blood leukocytes. In understanding the function of the lymphocytes, one must realize that most lymphocytes found in blood or lymph represent recirculating immunocompetent cells, i.e., cells that have developed the capacity to recognize and respond to antigens and are in transit from one lymphatic tissue to another. In the tissues associated with the immune system (Chapter 13), three groups of lymphocytes can be identified according to size: small, medium, and large lymphocytes, ranging in diameter from 6 to 30 pm. The large lymphocytes are either activated lymphocytes, which possess surface receptors that interact with a specific antigen, or natural killer (NIC) lymphocytes (see below). In the bloodstream, most lymphocytes are small or medium sized, 6 to 15 jLtm in diameter. The majority—more than 90%—are small lymphocytes.

In blood smears, the small lymphocyte approximates the size of a erythrocyte

When observed in the light microscope in a blood smear, the small lymphocyte has an intensely staining, slightly indented, spherical nucleus. The cytoplasm appears as a very thin, pale blue rim surrounding the nucleus. In general, there are no recognizable cytoplasmic organelles, other than an occasional fine azurophilic granule. The TEM reveals that the cytoplasm primarily contains free ribosomes and a few mitochondria. Other organelles are so sparse that they are usually not seen in a thin section. Small, dense lysosomes that correspond to the azurophilic granules seen in the light microscope are occasionally observed; a pair of centrioles and a small Golgi apparatus are located in the cell center, the area of the indentation of the nucleus.

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