Eosinophils have a circulating half-life of approximately 18 hours and a tissue life span of at least 6 days. They are capable of locomotion and phagocytosis and can enter inflammatory sites, but do so less readily than neutrophils. In tissues the primary location for eosinophils is in the epithelial barriers to the outside world such as, lungs, skin and GI tract. They are capable of returning to the circulating blood and bone marrow after they enter the tissues.
Eosinophils are active in parasitic infections and in allergic reactions such as asthma and hay fever, and may be present in great numbers in the peripheral blood during these conditions. Stress, shock, or burns may also cause an increase in this type of cell. Eosinophils modulate an allergic response by liberating substances which can neutralize mast cell and basophil products and also contain basic proteins which can be toxic to parasitic and mammalian cells.
The image on the right shows malarial ring forms, which are parasites, inside the red blood cells. This patient showed an increased eosinophil count due to his parasitic infection.
NOTE: Increases in eosinophil counts are more often associated with worm infections rather than malarial or protozoan infections. Additionally, eosinophils typically encounter the parasite in the tissues, not the blood stream.