The present invention relates to amino acid sequences that are directed against (as defined herein) any of IL-17A, IL-17F and/or IL-17A/F including combinations thereof, as well as to compounds or constructs, and in particular proteins and polypeptides, that comprise or essentially consist of one or more such amino acid sequences (also referred to herein as "amino acid sequences of the invention", "compounds of the invention", and "polypeptides of the invention", respectively).
