Diseased heart valve. Gross clinical specimen of a section through the human heart, showing stenosis (narrowing) of the bicuspid (mitral) valve. This narrowing is caused by rheumatic fever, resulting in the enlargment of the left atrium and back pressure on the blood flowing from the lungs. The bicuspid valve lies between the upper and lower (atrium & ventricle) chambers of the heart's left side. Rheumatic fever is caused by a bacterial infection (Streptococcus sp.), whose antigen (foreign body) is similar to the antigen on the muscle fibres of the heart. As a result of an immune response, antibodies attack the bacteria as well as the heart.