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In addition to the delayed-type hypersensitivity (DTH), a unique type of hypersensitivity could be induced at a late stage of the immune responses after hepatitis B surface antigen (HBsAg) immunization. This antigen-specific ear swelling that develops within 1 h after antigen challenge has been referred to as the early-type hypersensitivity (ETH) in contrast to the 24-h DTH. Although expression of ETH was earlier than DTH, the induction of the former needed 3 days longer than that of the latter. In ETH, the plasma protein leaked into the tissue and the vasopermeability increased within 15 min, causing the oedema of ETH. The observation that cyproheptadine, not dexamethasone, inhibited ETH suggests that it is mediated through the release of histamine and/or serotonin. Furthermore, ETH could be transferred by immune sera. Heat treatment (56 C for 4 h) did not destroy the transfer, suggesting that it was not mediated by IgE. The human anti-HBs sera from either hepatitis B virus infection or HBsAg vaccinee also contained the activity to transfer the ETH in mice.