The problems of major concern in immunohistochemical practice are discussed in the following order: (a) the mechanism of the Ag-Ab reaction in fixed tissue as opposed to the in vitro reaction; (b) the chemistry of fixation and its influence on the final result of the immunohistochemical reaction; (c) the various procedures used for antigen retrieval in formaldehyde-fixed tissue; and (d) the consideration of the possible mechanism underlying heat-induced antigen retrieval. Suggestions for further work to attempt a clarification of the mechanism involved in the Ag-Ab reaction in immunohistochemistry resorting to existing histochemical methods for the demonstration of protein side groups are presented, together with some examples already published.

The histochemical patterns of mucosubstances in 1.010 intestinal metaplasia (IM) foci in stomachs removed during surgery for benign ulcer and carcinoma has been studied. Two kinds of IM were characterized: the complete and the incomplete types, with the first one subdivided in Small intestine Type I, Small intestine Type II, and Colonic type, based on their similarity of mucosubstances, with those found in normal small intestine and colon, and the second one divided in two groups, depending on the presence or absence of sulfomucins. The incomplete type with sulfomucins was significantly more frequent in patients with carcinoma than in benign ulcer cases (P < 0.001) as well as, in stomachs bearing intestinal type of carcinoma than in stomachs with diffuse type of carcinoma (P < 0.005). The significance of the different types of IM, in relation to the gastric pathologic findings is discussed.

The development of calcitonin cells (C-cells) was investigated in rat thyroid glands from birth to 120 days, using an immunoperoxidase technique and a point-counting method. The proportion of C-cells to follicular cells was 4.5% on the day of birth and increased progressively to 10.4% by 120 days. The highest density of C-cells was noted in the mid-region of the lobes along a longitudinal axis. The caudal and cephalic regions of the lobes contained smaller numbers of C-cells. The C-cells tended to be more numerous in the posterior aspects of the lobes. Although the numbers of C-cells in 120-day-old animal were markedly increased as compared to animals at the time of birth, the cell distributions within the glands were similar at all ages.

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The simultaneous demonstration of lysozyme using the unlabelled antibody enzyme method and mucosubstances by staining with the periodic acid-Schiff (PAS) and Alcian Blue pH 2.5 techniques has led to the identification of a new subpopulation of mucus-producing cells containing lysozyme in small intestinal specimens from normal rats and from patients with inflammatory bowel diseases. These cells, containing both mucosubstances and lysozyme, are located in the lateral walls of the crypts of Lieberkuhn, but can occasionally be found also in or near the tips of the villi. The specific staining for lysozyme was observed in the apical and/or basal cytoplasm of these mucus-producing cells and was readily detected in sections counterstained for mucosubstances with Alcian Blue. The localization of these mucus-producing cells was similar both in the normal rat and in the pathological human specimens. Absorption controls and controls where a non-immune serum was substituted for the specific antilysozyme serum confirmed the specificity of the lysozyme localization.