Basophils are functionally related to, but not identical with, mast cells of the connective tissue (see Table 6.6, page 187). Both mast cells and basophils bind an antibody secreted by plasma cells, IgE, through high-affinity Fc receptors expressed on their cell surface. The subsequent exposure to, and reaction with, the antigen (allergen) specific for IgE triggers the activation of basophils and mast cells and the release of vasoactive agents from cell granules. These substances are responsible for the severe vascular disturbances associated with hypersensitivity reactions and anaphylaxis. Furthermore, both basophils and mast cells are derived from the same basophil–mast cell progenitor (BMCP) cell. If a specific BMCP expresses the granulocyte-related transcription factor CCAAT/enhancer-binding protein  (C/EBP), the cell becomes committed to differentiate into a basophil progenitor (BaP) cell. Basophils develop and differentiate in the bone marrow and are released to the peripheral blood as mature cells. In the absence of C/EBP transcription factor, a BMCP cell migrates to the spleen and after further differentiation travels as a mast cell precursor (MPC) to the intestine, where it becomes a mature mast cell.
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 (see Chapter 14, Lymphatic System), three groups of lymphocytes can be identified according to size: small, medium, and large lymphocytes, ranging in diameter from 6 to 30 m. The large lymphocytes are either activated lymphocytes, which possess surface receptors that interact with a specific antigen, or natural killer (NK) lymphocytes. In the bloodstream, most lymphocytes are small or medium sized, 6 to 15 m in diameter. The majority—more than 90%—are small lymphocytes.
In blood smears, the mature lymphocyte approximates the size of an erythrocyte.
When observed in the light microscope in a blood smear, small lymphocytes have an intensely staining, slightly indented, spherical nucleus (Plate 17, page 302). 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.
In the medium lymphocyte, the cytoplasm is more abundant, the nucleus is larger and less heterochromatic, and the Golgi apparatus is somewhat more developed (Fig. 10.11). Greater numbers of mitochondria and polysomes and small profiles of rough endoplasmic reticulum are also seen in these medium-sized cells. The ribosomes are the basis for the slight basophilia displayed by lymphocytes in stained blood smears.
Three functionally distinct types of lymphocytes are present in the body: T lymphocytes, B lymphocytes, and NK cells.