Plasma cells originate from terminally differentiated B cells. Plasma cells are oval or egg-shaped structures characterized by a stellate (star-like pattern) nucleus, nonstaining Golgi, and basophilic cytoplasm.
· The main function of the plasma cells is to produce and secrete all the classes of immunoglobulins into the fluids around the cells.
· They secrete thousands of antibody molecules per second, which are specific for the epitope of the antigen for a few days and then die.
· They, however, do not express membrane immunoglobulins.
· They divide very poorly, if at all, and are usually found in the bone marrow and in the perimucosal lymphoid tissues.
· They have a short lifespan of 30 days during which they produce large quantities of immunoglobulins.
Natural killer (NK) cells are morphologically described as large granular lymphocytes. These cells are called natural killer cells due to their ability to kill certain virally infected cells and tumor cells without prior sensitization. Their activities are not enhanced by exposure and are not specific for any virus. NK cells comprise approximately 5–10% of peripheral lymphocytes and are found in spleen and peripheral blood.
NK cells develop within the bone marrow and lack TCR, but possess another set of receptors called killer activation recep-tors and killer inhibition receptors. They also posses NK T cells, another subset of T cells, which share some functional char-acteristics with NK cells. These NK T cells unlike NK cells are stimulated by lipids, glycolipids, and hydrophobic peptides presented by a nonclassical class I molecule CD1D and secrete large amounts of cytokines, especially IL-4.
The main functions of the NK cells are to kill virus-infected cells and tumors. They do so by secreting cytotoxins, such as perforins and granzymes similar to those of cytotoxic T lym-phocytes and also by FasL-mediated apoptosis. They kill the viruses without presence of specific antibodies but by a mech-anism called ADCC. Both IL-12 and gamma interferons are potent activators of NK cells.
Granulocytes are a collection of white blood cells with seg-mented or lobulated nuclei and granules in their cytoplasm, which are visible with special stains. The granulocytes are classified as neutrophils, eosinophils, or basophils on the basis of cellular morphology and cytoplasmic-staining characteristics.
Both neutrophils and eosinophils are phagocytic, whereas basophils are not. Eosinophils play an important role in defense against parasitic infections, though their phagocytic role is significantly lower than neutrophils. Basophils, on the other hand, are nonphagocytic granulocytes that function by releasing pharmacologically active substances from their cyto-plasmic granules. These substances play a major role in certain allergic responses.
Mast cells are the other granulocytic cells that have a role in the immune system. These cells are found in a wide variety of tissues, including the skin, connective tissues of various organs, and mucosal epithelial tissue of the respiratory, genitourinary, and digestive tracts. Like circulating basophils, these cells have large numbers of cytoplasmic granules that contain histamine and other pharmacologically active substances. Mast cells, together with blood basophils, play an important role in the development of allergies.