CSF2 and pulmonary alveolar proteinosis: PAP can be categorized into: (I) primary PAP, which is caused by defects in the GM-CSF signaling axis that affects different stages of AM development as well as function and can be further divided into (I.I) autoimmune and (I.II) hereditary PAP; and (II) secondary PAP, which is mainly caused by reduced AM numbers due to other diseases.