Gut associated lymphoid tissue: a morphological and immunocytochemical study of the human appendix.

Gut associated lymphoid tissue in 15 normal appendices has been characterised in tissue sections using both morphological criteria and immunocytochemical techniques. A panel of monoclonal and polyclonal antibodies was used including antibodies to B-cells, T-cells, macrophages, HLA DR and immunoglobulins. The lymphoid tissue in the appendix was shown to bear a strong resemblance to that in lymph nodes with the exception of the region where the appendix follicles associate with the dome epithelium, which has no lymph node equivalent. This zone of cells between the lymphoid follicles and the dome epithelium termed the 'mixed cell zone' has been shown to contain an abundance of HLA DR-bearing cells, some of which have irregular nuclear morphology and resemble follicle centre cells. These cells were seen to extend into the epithelium of the dome but not the crypts. Using a monoclonal anti-B-cell antibody a population of B-cells was detected in the equivalent areas of mixed cell zone and epithelium and quantitative studies showed that these intraepithelial B-cells comprised approximately 4-5% of the cells in the epithelium. The mixed cell zone was also seen to contain T-cells, S-100 protein-containing macrophages and occasional lysozyme-containing macrophages. Plasma cells were rarely seen in this area.