The extrinsic allergic alveolites classically represent the immunological reaction of type III of the lower respiratory tract. In fact, all immunological mechanisms and several non specific factors are involved in these affections. If delayed hypersensitivity (type I) is today considered as part of the disease, the mechanisms involving reagins (type I) are also probably part of it but it is not so evident for type II reactions. Non specific factors are numerous, often hypothetical, because they are still experimental and not all enumerated. The activation of alveolar macrophages, the triggering of the complement system by another way, the enzymes found in dust or the irritation produced by inhaled products are all important factors.