A method for obtaining an immune response to a non-enteric pathogen antigen (NEPA) such as hepatitis B surface antigen (HBsAg) by feeding the antigen in a plant material to an animal that is immunoreceptive to the NEPA. It has now been discovered that the animal may be made immunoreceptive to the NEPA such as HBsAg by administering the plant material containing the NEPA in conjunction with a suitable adjuvant. The plant material is a substance comprising a physiologically acceptable plant material, especially potatoes, containing the NEPA, e.g. hepatitis B surface antigen (HBsAg). The NEPA, e.g. HBsAg in the plant results from expression by the plant of the NEPA due to genetic alteration.