Rheumatic fever (RF) MorphologyDuring acute RF, focal inflammatory lesions are found in various tissues. Distinctive lesions occur in the heart, called Aschoff bodies, which consist of foci of lymphocytes (primarily T cells), occasional plasma cells, and plump activated macrophages called Anitschkow cells (Pathognomonic for RF). These macrophages have abundant cytoplasm and central round-to ovoid nuclei in which the chromatin is disposed in a central, slender, wavy ribbon (hence the designation "caterpillar cells"), and may become multinucleated. During acute RF, diffuse inflammation and Aschoff bodies may be found in any of the three layers of the heart, causing pericarditis, myocarditis, or endocarditis (pancarditis). Inflammation of the endocardium and the left sided valves typically results in fibrinoid necrosis within the cusps or along the tendinous cords. Overlying these necrotic foci are small (1 - to 2- mm) vegetations, called verrucae, along the lines of closure. Subendocardial lesions, perhaps exacerbated by regurgitant jets, may induce irregular thickenings called MacCallum plaques, usually in the left atrium.Chronic Rheumatic Heart DiseaseThe changes of the mitral valve in are:Leaflet thickening Commissural fusion Shortening, thickening and fusion of the tendinous cordsIn chronic disease, the mitral valve is virtually always involved The mitral valve is affected alone in 65% to 70% of cases, and along with the aortic valve in another 25% of cases. Fibrous bridging across the valvular commissures and calcification create "fish mouth" or “buttonhole” stenoses.