Eosinophil - a type of white blood cell with important implications in allergy
Mammalian blood consists of red blood cells, platelets and five distinct types of white blood cell (WBC), one of which is the eosinophil, all floating around in a rich fluid called plasma. Red blood cells are concerned with oxygen transport, and WBCs provide the body with the ability to fight infection and to provide long term immunity.
The appearance of the nuclei of white blood cells subdivides them into two clear groups, mononuclear leukocytes (monocytes and lymphocytes) and polymorphonuclear leukocytes or granulocytes. Eosinophils belong in the latter group along with basophils and neutrophils, and they make up 1-4% of the total WBC count of a healthy individual.
The nucleus of the eosinophil consists of 2 lobes connected by a thin thread of chromatin. The cytoplasm surrounding the nucleus is packed with tiny spherical granules which stain orange/red when samples are treated with the dye eosin, hence the name eosin-o-phil or eosin-loving.
Each type of WBC has its own unique function in the fight against disease. The number of eosinophils in the blood stream is normally quite low, suggesting that perhaps it may not be quite so important as other types of WBC. The proportion does rise, however, during certain allergic reactions such as asthma and hay fever (which is bad), and where there is infection by certain types of parasite, particularly parasitic worms (which is pretty useful).
When parasites are present in the body, chemical messages are released by lymphocytes and neutrophils that stimulate the production and migration of eosinophils from the bone marrow to the site of infection.
Eosinophils have receptors on their cell membranes which attach to the surface of the parasitic worm. Once attached, the signal is given to realease the contents of their granules - and hopefully it's goodbye parasite! The granules contain an array of nasty chemicals, including peroxidase, photolipase, phospholipase and cationic major basic protein that attack the parasitic membranes, dissolving them on contact.
Eosinophils and allergy
Unfortunately for sufferers, allergic reactions are the body's misguided attempts to eliminate 'invaders' - in the case of allergy these invaders are not actually harmful in themselves but the body's reaction to them can be at the very least uncomfortable and at worst deadly.
During an allergic reaction, eosinophils are drawn to the site of the supposed 'infection' and once there the chemicals released sustain and enhance the inflammatory process. Some of the chemicals directly damage the surrounding tissue, while cytokines recruit other inflammatory cells (including mast cells and more eosinophils) to the site, resulting in a vicious chain reaction and an acute allergic attack.
The presence of eosinophils and their chemical products is significantly higher in the sputum and nasal secretions of asthma and hay fever sufferers, and can be used as an aid to diagnosis. The over-reaction of eosinophils can be suppressed by steroid drug therapy but research is underway to find other antagonists of the eosinophil pathway.
Other diseases associated with high eosinophil levels: