As a student of history and social science for more than 50 years, I have a long second-hand acquaintance with anti-Semitism (really anti-Jews, a point which matters when Arabs are Semites too). Thanks in the first place to Daniel Finklestein, I learned that German districts with a history of medieval pogroms against Jews produced especially strong votes for Nazis 600 years later. A warning to conservatives about passing on traditions if ever there was one.
For myself, one of the very reasons why I still support a two-state solution to the Palestinian question, and oppose expansion of Jewish settlements, is to avoid reopening the blood curse of anti-Semitism. I confess I am not sufficiently familiar with the internal affairs of the British Labour Party to know how much of a real emergence of anti-Jew sentiment there is, rather than just political insult trading. But insult trading has a nasty ability to turn into reality.
For his own interests, Jeremy Corbyn needs to get a grip on the anti-Semitism issue immediately. I am not sure about the wisdom of the International Holocaust Remembrance definition as such, but a clear exposition of the distinction between legitimate (and civil) criticism of Israeli policy and racist anti-Semitism is essential. This is one case where treating us as a bunch of morons is appropriate. We all think we know the difference between critical thought and racist abuse, but real racists always have an interest in making sure we don’t see it.