IL17R_fnIII_D1 is the first of two fibronectin 3-like domains on interleukin-17 receptor proteins A and B. The tow fnIII domains are linked and together bind two molecules of IL-17 at one of its receptor-binding interfaces. This allows the other interface to bind to another receptor, thus allowing the IL-17 family of homodimeric cytokines to coordinate two different receptors [1].

1: Structural basis of receptor sharing by interleukin 17 cytokines. Ely LK, Fischer S, Garcia KC; Nat Immunol. 2009;10:1245-1251. PMID:19838198