There is provided according to the invention a method of treating a mammal suffering from or susceptible to an immune reaction to drug treatment comprising the raising of anti-drug antibodies which method comprises (a) ex-vivo treating antigen presenting cells obtained from the mammal with an agent which induces IDO in said antigen presenting cells in the presence of said drug or an epitope containing fragment thereof and (b) after IDO has been induced in said antigen presenting cells, transferring said cells back to the mammal thereby to establish immune tolerance to the drug.
