The present invention relates to immunoreactive compositions and methods for immunizing humans or animals against sterols, such as cholesterol and its derivatives, and their use in methods for reducing the serum cholesterol levels of the immunized individuals and to retard or reduce the severity of atherosclerosis or atherosclerosis plaques caused by ingestion of dietary cholesterol. Another embodiment of the present invention encompasses ergosterol or ergosterol derivative compositions useful for the treatment or prevention of fungal infection, and methods of use thereof. Yet another aspect of the invention is dairy products containing anti-ergosterol antibodies produced by dairy animals immunized against ergosterol according to the present invention and methods of making thereof. A further aspect of the invention is a diagnostic assay for determining whether a human or animal has a fungal infection.