Expression of class I and II human leukocyte antigens by thyrocytes and lymphocytic infiltration on human thyroid tumors. An immunofluorescence study. Surgical thyroid sections from 30 papillary carcinomas (PC), six medullary carcinomas (MC), three anaplastic carcinomas (AC), two follicular carcinomas (FC), and 16 adenomas (AD) were examined with an indirect immunofluorescence technique employing different monoclonal antibodies to evaluate the expression of human leukocyte antigen (HLA)-A, B, C (Class I) and DR, DP, DQ (Class II) by thyrocytes, together with the phenotype and distribution of inflammatory cells. Ten PC and four FC were also investigated for the presence of intercellular adhesion molecule-1 (ICAM-1). In situ deposits of immunocomplexes and circulating thyroid autoantibodies were also evaluated. An increased expression of Class I antigens was found in all PC and FC, in 33% of MC and AC, and in 31% of AD. An anomalous expression of Class II antigens was observed in 70% of PC, in 50% of FC, in 33% of AC, in 19% of AD, and in none of the MC. Expression of DP or DQ was revealed only in a portion of the DR-positive glands. A reduction of microsomal autoantigen expression was found. No ICAM-1-positive thyrocytes were detected. A moderate T-lymphocytic infiltrate was noticed only in PC, where it was correlated with DR and DP and/or DQ coexpression. B-cells and natural killer cells were virtually absent. The authors speculate that the weak Class II antigens expression, together with the partial or complete loss in microsomal autoantigen and the absence of ICAM-1 by thyrocytes, may account for the limited engagement of immunocompetent cells observed in thyroid tumors. 