HLA (human leucocyte antigen) molecules, which were initially studied because of their importance in organ transplant rejection, have been found to play a fundamental role in immunity, with the physiological role of presenting peptide antigens to T lymphocytes. HLA class I molecules present peptides to cytotoxic (CD8+) T lymphocytes, which are important in defence against viruses and other intracellular pathogens, whereas HLA class II molecules present peptides to CD4+ T cells, which are important in generating antibody and cellular responses mainly to antigens outside the cell. HLA molecules are thus central to immune defence against infecting micro-organisms and to the development of autoimmunity. High resolution characterisation of HLA alleles by DNA methods has recently become feasible.1

Trachoma is the leading infectious cause of blindness. There is evidence that the clinical features both of active inflammatory disease and of the cicatrising sequelae are immunopathologically mediated. In endemic areas, familial clustering of active trachoma cases occurs, …