1) Treatment of disease by the administration of substances which produce a biological reaction in the organism. It includes the use of sera, antitoxins, vaccines, cells, tissues, and organs. (From Dorland, 28th ed)

2) Treatment to stimulate or restore the ability of the immune system to fight infection and disease. Also used to lessen side effects that may be caused by some cancer treatments. Also known as immunotherapy, biotherapy, or biological response modifier (BRM) therapy.