An inkjet ink set comprising at least two inks, wherein at least one ink contains a betaine compound and at least one other ink contains a nonionic surfactant. This invention is also concerned with IL-4 production inhibitors, antiallergic agents and atopic dermatitis preventives and improvers, each of which contains an essential oil such as cedarwood oil. The method further includes encrypting the message using a recipient public key and a plug-in secret key provided by the plug-in before sending the message.