Google and other search engines have been on the receiving end of a dozen or so lawsuits challenging a common practice in search advertising: the use of someone’s trademark, say Mercedes Benz, to show an ad for a competitor, say a BMW dealer. In that case, the BMW dealer would be buying ads tied to the search keyword “Mercedes Benz” from Google.

So far the legal scorecard favors search engines. They have won several of these cases, and while they have suffered some adverse rulings they have yet to definitively lose a case.

Now American Airlines is suing Google over this very issue in federal district court in Texas. (Here’s the complaint in a 4-megabyte PDF file, from the site of the Santa Clara University Law School.)

I asked Eric Goldman, a Santa Clara University law professor and an expert on these issues, whether anything about this case stands out. His answer: A plaintiff with a lot of trademarks, deep pockets and the seeming willingness to fight. As Mr. Goldman noted in his blog, American Airlines said it was not bringing this lawsuit lightly.

“Google’s search engine is helping third parties to mislead consumers and misappropriate the American Airlines Marks by using them as ‘keyword’ triggers for paid advertisements and by using them within the text or title of paid advertisements,” the complaint says.

Given prior rulings in other courts, a defeat for Google may not be likely. But it would be devastating, as the use of trademarks as keywords is an integral part of search marketing. “They can’t afford a loss on any of this,” Mr. Goldman said.

Mr. Goldman’s blog offers further details on the case, as well as the legal precedent and his analysis of some of the challenges American Airlines could face.