What is Eosinophils – Normal Count, Functions, Origin & Life Span

Presentation Notes Eosinophils Definition, High Low Count, Development Role in Human Blood for medical & Biology students in word / .doc / ppt format


Eosinophils have a diameter of 10 to 15  and account for 1-4% of the WBCs of normal persons. Usually (but not invariably) the nucleus of the eosinophil is bilobed. Cytoplasmic granules are coarse eosinophilic (orange red) under Leishman's stain but in the class rooms they (the granules) often look muddy colored.
Eosinophilic granules contain, (i) major basic protein(MBP), (ii) eosinophil peroxidase (EPO) and (Hi) cationic protein

Kinetics, life span, origin and development

Eosinophils are born from the committed stem cells called, CFU-Eo . From CFU-Eo, different steps of maturation  are : CfU-Eo -. myeloblast -. pro­myelocyte -. myelocyte -. metamyelocyte -. band form -.mature eosinophil. In the peripheral blood only mature eosinophils and occasional band forms may be seen in normal persons. 'Myeloblasts etc, in the peripheral blood are found only in abnormal conditions.Note, by Leishman's stain, one practically cannot differ­entiate between neutrophilic, eosinophilic or basophilic myeloblast..

In the bone marrow (RBM), the eosinophil stays for about 5 or 6 days -. it then enters the circulatory pool and stays for 8-12 hrs -. then it emigrates to tissue (= tissue pool), mostly skin and subcutanous tissue, where it dies and is then removed. From the tissue pool, recirculation of eosinophil (entry of eosinophil in the circulatory pool) occurs. Blood pool (intravascular pool) has two components  : (i) circulatory pool and (il) the marginal pool.

Functions of eosinophils are not clearly known. However , the clearly known facts are: (i) in bacterial infections, unlike the neutrophils, eosinophilic count does not, rise, but (ii) the eosinophilic count distinctly rises when : (a) there is parasitic infestation (eg, filariasis/hook worm round worm infestation) or (b) in such allergic conditions 'like bronchial asthma/hay fever etc; (c) besides there is an entity, idiopathic hypereosinophilic syndrome, where no dis­cernible cause of hypereosinophilia is known. On the other hand, count of eosinophil falls '(eosinopenia) after glucocorticoid therapy. It is believed eosinophils have a role in (i) containment in parasitic infections and (ii) in allergic inflammation.

The MBP of the eosinophilic granules has histaminase (ie, histamine destroying) activity. Histamine is one of the agents which produce allergic reactions. Eosino­phil peroxidase, on the other hand, may facilitate histamine secretion from the mast cells. It is possible eosinophils kill the parasites and T cell accentuates the ability of the eosinophils to kill the parasites. Eosinophils are poorly phagocytic and the EPO (Eosinophilic peroxidase) has some antibacterial action.

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