Here we will discuss autoimmune diseases and diseases based on a hyperreactive immune system, so-called allergic diseases. A few other diseases based on gene mutations will be included here, as they mimic autoimmune diseases. In autoimmune-induced interstitial lung disease many different factors come together: A wide variety of immune reactions can cause a wide variety of tissue reactions, for example circulating autoantibodies either capable or devoid of complement activation, circulating immune complexes including large insoluble immune complexes formed by idiotypic-antiidiotypic antibody networks, activation of coagulation, metabolism of pro-inflammatory substances, involvement of different types of leukocytes, and not the least, drugs given for the relieve of symptoms. These drugs themselves can cause toxic or inflammatory side effects. Another aspect of autoimmune diseases lies within its dynamic: An acute phase is changed into a phase with declining symptoms, going into a resolving stage, again starting acute, but also can progress into a subacute and chronic phase. Each of these phases will be accompanied by a different histology. Acute hypersensitivity pneumonia/extrinsic allergic alveolitis and asthma have already been discussed in previous chapters; however, chronic hypersensitivity pneumonia is included here, as it can present as fibrosing pneumonia. Allergic mycosis and drug allergies are also discussed.