Arendse et al. (2005) show that Leishmania major infection increases IL9-production in lymph node and splenocytes culture supernatant stimulated with promastigotes antigen at early infection stage in susceptible BALB/c mice, but not in resistant C57bl/6 mice, indicating that IL-9 acts as a susceptibility factor in leishmaniasis stimulating an adverse Th2-response in BALB/c mice. This evidence concerns the gene IL9 and infection.