THE Mycobacterium bovis strain BCG shares antigenic components with several neoplasms, including human malignant melanoma1-5. Other species of bacteria also have antigenic similarities with a guinea pig hepatoma 6. Antibodies to mycobacteria and other bacterial antigens are universally present in sera from normal humans7,8. Since there is a broad range of shared or cross-reactive antigens among mycobacteria and unrelated microorganisms9, it is conceivable that an immune response by normal humans to mycobacteria has been induced by exposure to taxonomically unrelated microorganisms ubiquitous in the environment. Cytotoxic reactions by lymphoid cells from normal animals and humans to many kinds of tumour cells have been reported10-15. Antibodies in sera from patients with various neoplasms have often been observed that react with antigens derived from their own tumours or from those of the same histological type16-18.Humoral responses to tumour-associated antigens have also been noted in sera from normal humans, gibbons and mice19-21. We have investigated whether sera from normal, tumour-free humans contain antibodies which can react with melanoma-associated as well as with BCG antigens. If this were the case, perhaps immunological reactions by normal humans to certain tumours are widespread and reflect previous sensitisation by microorganisms.
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