Substantially human antisera are provided by genetically modifying a domestic animal generally weighing at least about 1 kg. The domestic animal is genetically modified by generating inactive heavy and light chain immunoglobulin loci and integrating at least functional portions of the human heavy and light chain immunoglobulin loci, whereby the human loci generate an immune response. The antisera find use in the treatment of diseases, immunocompromised patients and in case of transplantation.