Pathological changes of bacterial pneumonia A. Lobar pneumoniaLarge confluent area of the lung or entire lobes is consolidated. The lower lobes are affected most commonly. There are four stages of the inflammatory response (Laennec's stages) -1. Stage of congestion (initial phase)The affected lobe is enlarged, heavy, dark red and congested. Cut surface exudes blood-stained frothy fluid. There are dilatation and congestion of alveolar capillaries. There are few neutrophils and numerous bacteria in the alveolar fluid.2. Stage of red hepatization (early consolidation)The term hepatization refers to the liver-like consistency of the affected lobe on cut section. The affected lobe is red and firm. The edema fluid of preceding stage is replaced by strands of fibrin. There is marked cellular exudate of neutrophils with extravasation of red cells.3. Stage of gray hepatization (late consolidation)The affected lobe is grayish brown, firm and dry. The fibrin strand are dense and more numerous. There is progressive disintigration of red cells and neutrophils. The macrophages begin to appear in the exudate. The organisms are less numerous and appear as degenerated forms.4. ResolutionThe previously solid and fibrinous constituent is liquefied by enzymatic action. Granular and fragmented strands of fibrin in the alveolar spaces are seen due to progressive enzymatic digestion. There is progressive removal of fluid content as well as cellular exudate from the air spaces, resulting in restoration of normal lung parenchyma with areation.B. BronchopneumoniaPatchy areas of red or grey consolidation, more often multilobar and frequently bilateral and basal (lower zones) because of tendency of secretions to gravitate into lower lobes. There is suppurative exudate, consisting chiefly neutrophils, filling bronchi, bronchioles and adjacent alveolar spaces. Alveolar septa thicken due to congested capillaries and leucocytic infiltration.