"As their name suggests, agranulocytes lack visible granules in the cytoplasm. Agranulocytes can be categorized as lymphocytes or monocytes. Among the lymphocytes are natural killer cells, which play an important role in nonspecific innate immune defenses. Lymphocytes also include B cells and T cells and are central players in specific adaptive immune defenses. Monocytes differentiate into macrophages and dendritic cells, which are collectively referred to as the mononuclear phagocyte system. Most lymphocytes are primarily involved in the specific adaptive immune response. An exception is the natural killer cells (NK cells); these mononuclear lymphocytes use nonspecific mechanisms to recognize and destroy cells that are abnormal in some way. Cancer cells and cells infected with viruses are two examples of cellular abnormalities targeted by NK cells. Recognition of such cells involves a complex process of identifying inhibitory and activating molecular markers on the surface of the target cell. Molecular markers that make up the major histocompatibility complex (MHC) are expressed by healthy cells as an indication of “self.” NK cells can recognize normal MHC markers on the surface of healthy cells, and these markers serve as an inhibitory signal preventing NK-cell activation. However, cancer cells and virus-infected cells actively diminish or eliminate expression of MHC markers on their surface. When MHC markers are diminished or absent, the NK cell interprets this as an abnormality and as a cell in distress. This is one part of the NK-cell activation process. NK cells are also activated by binding to activating molecular molecules on the target cell. These activating molecules include “altered self” or “nonself” molecules. When an NK cell recognizes a decrease in inhibitory normal MHC molecules and an increase in activating molecules on the surface of a cell, the NK cell is activated to eliminate the cell in distress."