House explains Finn got the wrong blood type because an autoimmune disease made him look type AB, and finally has a case of lupus.
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Every House moment featuring Gregory House, in story order. Played by Hugh Laurie.

Every House moment featuring Eric Foreman, in story order. Played by Omar Epps.

Every House moment featuring Chris Taub, in story order. Played by Peter Jacobson.

Every House moment featuring Thirteen, in story order. Played by Olivia Wilde.

Every House moment featuring Lawrence Kutner, in story order. Played by Kal Penn.

Every House moment featuring Amber Volakis, in story order. Played by Anne Dudek.