 Specific granules of eosinophils contain a crystalloid body that is readily seen with the TEM, surrounded by a less electron-dense matrix. These crystalloid bodies are responsible for the refractivity of the granules in the light microscope. They contain four major proteins: an arginine-rich protein called major basic protein (MBP), which accounts for the intense acidophilia of the granule; eosinophil cationic protein (ECP); eosinophil peroxidase (EPO); and eosinophil-derived neurotoxin (EDN). MBP is localized in the crystalloid body; the other three proteins are found in the granule matrix. MBP, ECP, and EPO have a strong cytotoxic effect on protozoans and helminthic parasites; EDN causes nervous system dysfunction in parasitic organisms; histaminase neutralizes the  activity of histamine; and arylsulfatase neutralizes leukotrienes secreted by basophils and mast cells (see Chapter 6, Connective Tissue,  FOLDER 10.5 Clinical Correlation: Hemoglobin Breakdown and Jaundice
If the conjugation of bilirubin or its excretion into bile by the liver cells is inhibited, or if blockage of the bile duct system occurs, bilirubin may reenter the blood, causing a yellow appearance of the sclera of the eye and the skin. This con-dition is called jaundice. Jaundice can be caused by the destruction of circulating erythrocytes. An example of such a condition is a hemolytic transfusion reaction when ABO-incompatible blood is administered to a patient, usu-ally because of a clerical error. Massive hemolysis of the transfused erythrocytes may be associated with severe systemic complications such as hypotension (decreased blood pressure), renal failure, and even death. Jaundice is also characteristic in a variety of hemolytic anemias that result from either inherited defects in the erythrocyte (e.g., hereditary spherocytosis) or external fac-tors such as pathogenic microorganisms, animal venoms, chemicals, and drugs. Some jaundice is common in new-born infants (physiologic jaundice) because of ineffi-ciency of the bilirubin-conjugating system in the newborn liver.
page 186). Specific granules also contain histaminase, arylsulfatase, collagenase, and cathepsins.
 Azurophilic granules are lysosomes. They contain a variety of the usual lysosomal acid hydrolases and other hydrolytic enzymes that function in destruction of parasites and hydrolysis of antigen–antibody complexes internalized by the eosinophil.
Eosinophils are associated with allergic reactions, parasitic infections, and chronic inflammation.
Eosinophils develop and mature in the bone marrow. Once released from the bone marrow, they circulate in peripheral blood and then migrate to the connective tissue. Eosinophils are activated by interactions with IgG, IgA, or secretory IgA antibodies.
The release of arylsulfatase and histaminase by eosinophils at sites of allergic reaction moderates the potentially deleterious effects of inﬂammatory vasoactive mediators. The eosinophil also participates in other immunologic responses and phagocytoses antigen–antibody complexes. Thus, the count of eosinophils in blood samples of individuals with allergies and parasitic infections is usually high. Eosinophils play a major role in host defense against helminthic parasites. They are also found in large numbers in the lamina propria of the intestinal