The conformation of a peptide defines its antigenic specificity. In most cases, a free peptide is in a random form, whereas the same peptide within a protein has a defined conformation. Immunological cross-reactions between the two are rare. Nevertheless, there are cases where an anticonformational antibody may transconform (change the conformation of) the free peptide, allowing the reaction to occur. Based on such considerations, synthetic vaccines against infectious diseases may be constructed. The same is true for autoimmune diseases, where, at least in one case-that of the exacerbating-remitting type of multiple sclerosis-we have developed a polypeptidic drug-vaccine, copolymer 1 (denoted Copaxone), capable of suppressing the autoimmune phenomena in a specific way. This drug-vaccine has been approved for treatment by the US Food and Drug Administration (FDA).