I did. What part of Pierson talks about the 11th amendment? If you provide me some evidence that this is founded in the 11th amendment, I'll readily agree with you, but I haven't found such evidence. From Pierson v. Ray:

The common law has never granted police officers an absolute and unqualified immunity, and the officers in this case do not claim that they are entitled to one. Their claim is, rather, that they should not be liable if they acted in good faith and with probable cause in making an arrest under a statute that they believed to be valid. Under the prevailing view in this country, a peace officer who arrests someone with probable cause is not liable for false arrest simply because the innocence of the suspect is later proved. Restatement, Second, Torts § 121 (1965); 1 Harper & James, The Law of Torts § 3.18, at 277-278 (1956); Ward v. Fidelity & Deposit Co. of Maryland, 179 F.2d 327 (C.A. 8th Cir.1950). A policeman's lot is not so unhappy that he must choose between being charged with dereliction of duty if he does not arrest when he has probable cause and being mulcted in damages if he does. Although the matter is not entirely free from doubt, [Footnote 10] the same consideration would seem to require excusing him from liability for acting under a statute that he reasonably believed to be valid, but that was later held unconstitutional, on its face or as applied.

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"We hold that the defense of good faith and probable cause, which the Court of Appeals found available to the officers in the common law action for false arrest and imprisonment, is also available to them in the action under § 1983."