CST4 and bacterial infectious disease: Cystatins and serpins were also upregulated in I. scapularis that were infected with A. phagocytophilum and in Ixodes ricinus challenged with Bartonella hensela. On the other hand, several serpins, identified as components of the D. variabilis immune system, were found to be unregulated on a transcription level in response to bacterial infection [23].