The probability of infection with fungi, as well as parasitic nematodes or arthropods may increase in overcrowded population of animals and human. The widespread overuse of drugs and immunosuppressants for veterinary or medical treatment create an opportunity for many pathogenic species. The aim of the review is to present the common molecular characteristics of such pathogens as fungi and nematodes and other chitin bearing animals, which may both activate and downregulate the immune response of the host. Although these pathogens are evolutionary distinct and distant, they may provoke similar immune mechanisms. The role of chitin in these phenomena will be reviewed, highlighting the immune reactions that may be induced in mammals by this natural polymer.