Hepatization is conversion into a substance resembling the liver; a state of the lungs when gorged with effused matter, so that they are no longer pervious to the air. Red hepatization is when there are red blood cells, neutrophils, and fibrin in the pulmonary alveolus/ alveoli; it precedes gray hepatization, where the red cells have been broken down leaving a fibrinosuppurative exudate. The main cause is lobar pneumonia.