EOSINOPHILS
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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