Anaesthesia by halothane and puncture of the jugular vein through a small skin incision gives good blood samples with narrower normal ranges than those usually quoted, especially for leucocyte counts.
IT has been postulated that eosinophil leucocytes respond chemotactically to histamine (Archer, 1956(Archer, , 1957, and some evidence has been adduced that the cells contain an antihistamine compound (Kovds, 1950 ; Kovahs and JuhBsz, 1951-52 ; Vercauteren, 1953-54), but this compound has not been finally identified. There is a remarkably active physiological mechanism in the intact animal which has the effect of limiting or rapidly disposing of abnormally high histamine concentrations and it therefore seems unlikely that eosinophil leucocytes are functionally concerned primarily with histamine detoxication. 5-hydroxytryptamine is released when histamine is released in vivo (Parratt and West, 1957 ; Waalkes, Weissbach, Bozicevich and Udenfriend, 1957). An investigation into the eosinophilic response to modifications in concentration of histamine and of 5-hydroxytryptamine has therefore been undertaken.It is exceedingly difficult to produce modifications in the whole body concentration of these agents with subsequent haematological investigations of the circulating blood, but it is relatively easy to alter these concentrations locally in a specific tissue. A study has, therefore, been made upon blood cells in skin because the latter is a readily removable tissue and easy to modify locally in the intact animal. I n the horse, eosinophil leucocytes have particularly large and wellstaining granules and they can be readily demonstrated in stained sections. Unfortunately mast cells are relatively scanty in horse skin and it has not been possible to make satisfactory parallel studies on these cells ; in any case, these are cells that are fixed in tissue, and only decreases in their number could be assayed. MATERIAL AND METHODSCross-bred ponies were used. Skin sites were shaved and thus marked upon the lateral aspect of the neck. About 12 sites on each side of the neck could be prepared so as to maintain a minimum distance of 4 in. (10 om.) between site centres. All injections were given intradermally and a standard volume of 0.2 ml. per injection was adopted throughout. Injections at the same site were made daily at 10 a.m. for four days and at 11 a.m. on the fifth day. Local anssthesia was produced by injection of 2 per cent. procaine subcutaneously in a line at least 2 in. (5 em.) dorsal to each biopsy site. Skin biopsies were taken at 3 p.m. on the fifth day, i.e. 4 hr after the last intradermal injection, with a rotary punch, according to the technique described by Evans, Nisbet 95
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