<a href="https://en.wikipedia.org/wiki/Lloyd_J._Old" id="mwARo" rel="mw:WikiLink" title="Lloyd J. Old">Lloyd J. Old</a> discovers the first linkage between the <a href="https://en.wikipedia.org/wiki/Major_histocompatibility_complex" id="mwARs" rel="mw:WikiLink" title="Major histocompatibility complex">major histocompatibility complex</a> (MHC) and disease—mouse leukemia—opening the way for the recognition of the importance of the MHC in the immune response.