Immunohistochemical study of 22 cases of thymoma.

The histological and clinical features of 22 thymomas were reviewed. Immunohistochemical studies were performed using antibodies to cytokeratins (CAM 5.2 and S29) and to desmosomal protein. The heterogeneity of staining patterns seen in the epithelial cells supported the concept of separating thymomas into cortical, medullary, or mixed groups. Interdigitating cells were identified by antibody to S100 protein, and these varied in number between different tumours. Clustering of interdigitating reticulum cells occurred in association with foci of mature lymphocytes which were shown by staining of the leucocyte common antigen (CD45). The extent to which this occurred was used to assess the degree of medullary differentiation within the thymomas and this was correlated with the histological and clinical features. The lymphocyte population of six of the thymomas was studied using a range of antibodies to T and B cells; this showed the presence of B lymphocytes in thymomas.