IL17A and periodontitis: On the one hand, IL-17 has been demonstrated to protect against extracellular pathogens (Khader et al., 2009; Hernandez-Santos and Gaffen, 2012) and can trigger the generation of antimicrobial peptides (Liang et al., 2010), which are assumed to be protective in periodontitis (Diamond et al., 2008; Gorr, 2009).