I think that anti-Semitism is a qualitatively distinct form of racism which ought to be considered on the borderline of mental illness. I'll admit fault for calling it a mental illness without qualification.

Let's be clear we're talking about the same thing here. The definitions for mental illness that I'm familiar with say that mental illness must be something that is not widespread in the person's culture, a beliefs or behavior that others consider weird or irrational. People imitate and conform to other's beliefs and actions so much, that anything that is common to a large segment of the population (e.g. religious belief) cannot be usefully called a mental illness. Anti-Semitism clearly fails this test.

Anti-Semitism is not only racial but also religious and nationalist, and it can happen anywhere.

Hating outgroups based on religious and nationalist lines, is just as normal and widespread as on racial lines. Almost every multi-religious society has or had in the past a large degree of segregation, distrust, and perhaps sectarian violence. The same goes for populations of "mixed nationalities".

Since the Jews historically lived among people where they were at once a religious, racial, and (in the last century) nationalist outgroup, it is not at all surprising that they were hated. Just like, since U.S. blacks are mostly a distinct social class from whites, and were previously a legally distinct class too, it's natural for this distinction to merge with the racial hatred and make it stronger.

We use a special term, anti-Semitism, because of the its historical importance, but it doesn't seem to me to be qualitatively different from other kinds of inter-group hatred.

Garden-variety racists are not typically racialists with a well-constructed theory to support their bigotry, but anti-Semites almost always are.

This has only stood out since the 19th century in Europe. (Previously, other societies concerned themselves with racial purity and descendants of Jews, like Christian Spain; but they were the exception, not the rule.)

Yet Anti-Semitism has existed as long as mainstream Christianity. (And probably before - I just don't happen to know anything about the integration or otherwise of Jews in the Roman and Greek worlds.) Anti-Semitism changed a little in character when racial theories were added to the mix, but the so-called "modern" A-S could not have existed (in such a magnitude) with the millenia of "classic" A-S preceding it.

Conspiracies about Chinese and Japanese subterfuge wax and wane, but anti-Semitism stays. Jews are blamed simultaneously for the worst excesses of capitalism and socialism, for the kidnap and murder of children for ritual, food, and sport. They are out to undermine the true religion and dilute the blood of the best races, and turn the nations into beggars.

I would like to see a quantitative survey of the equivalent of blood libels in other famous sectarian hatreds. I would expect to find out that Christians have told (and tell) just as bad tales about Muslims, Protestants about Catholics, US whites about blacks, as anyone has told about Jews.

Also, A-S tales are famous in our culture. Mostly everyone has heard of the Blood Libel and the Protocols of the Elders of Zion. Maybe we just haven't heard enough non-A-S examples, and so A-S has become highly available to our thinking.