Élie Metchnikoff
The first report of immunizing an animal of one species (Guinea pig) against the immune cells of another species (mouse lymphocytes) was by Élie Metchnikoff in 1899. He reported injecting cells recovered from mouse lymph nodes into Guinea pigs and waiting for the immunization to result in the accumulation of anti-mouse antibodies in the Guinea pig blood. When he subsequently collected serum from these Guinea pigs and injected it into normal mice he observed a marked depletion in the number of circulating mouse lymphocytes.