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The immune system of patients who are infected with human T-cell leukemia virus type I (HTLV-I) and have developed adult T-cell leukemia (ATL) is suppressed. However, the status of the immune system in carriers of HTLV-I, who are healthy and have no disease symptoms, is not certain. Previous studies have shown that the immune reaction to the tuberculin skin test, which measures delayed-type hypersensitivity (DTH), is suppressed in HTLV-I carriers in Japan, where HTLV-I infection is endemic. The suppression is more pronounced in individuals who are elderly, aged 60 years or older. Other types of DTH, reaction to purified protein derivative (PPD) of tuberculin and phytohemagglutinin (PHA), were examined in 56 patients, aged from 62 to 93 years old. The subjects were hospitalized with diseases that did not cause a suppression of the immune system. Twenty-two (39 percent) had antibodies to HTLV-I, indicating infection, but none had symptoms of disease caused by HTLV. There was an increased level of nonreactivity to PPD and PHA with increasing age. The degree of nonreactivity was greater in carriers than in non-carriers. Eighty-eight percent (15 out of 17) of HTLV-I carriers were nonreactive to both antigens, compared with 60 percent (15 out of 25) of non-carriers. Among the carriers who reacted to the antigens, the degree of reaction to PPD was reduced, but reaction to PHA was not reduced. These results confirm the finding that older individuals who are carriers of HTLV-I have reduced DTH response. (Consumer Summary produced by Reliance Medical Information, Inc.)