Answer: b, c, d Eosinophils constitute 1% to 3% of the leukocyte population of the bloodstream. They also reside in tissues and they exhibit phagocytic capabilities. They are less effective as bactericidal cells than neutrophils, but play a major role in defense against parasites. Eosinophils are primary effectors in allergic reactions by virtue of IgE receptors (which are not found on neutrophils). Basophils are fully differentiated cells released into the circulation from bone marrow. Basophils are the major, if not sole, source of histamine in the blood. Histamine is a vasoactive amine and the major mediator of the IgE-mediated immediate hypersensitivity response. Histamine release from basophils is induced by complement products as well as by IgE receptors. Mast cells are formed from bone marrow precursors that differentiate and proliferate in connective tissue. Mast cell granules contain histamine and proteoglycans. They represent the major source of histamine in most tissues except the stomach and central nervous system. The monocyte–macrophage system consists of phagocytic cells scattered throughout the body. During acute inflammation, monocytes respond to chemoattractants released and are recruited to the site of inflammation. Mononuclear phagocytes respond to inflammatory stimuli by releasing M-CSF, GM-CSF, IL-1, and TNF, in addition to a variety of growth factors. These factors increase the production of mononuclear phagocytes and several of these factors enhance the ability of effector cells to respond to chemotactic stimuli released at the site of injury. Thus, the mononuclear phagocytes are important in initiating and augmenting the cycle of events that result in recruitment and activation of inflammatory cells at sites of inflammation