MEDizzy - A 25-year-old woman presents with fever, malaise, headaches,

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Respiratory System

A 25-year-old woman presents with fever, malaise, headaches, and muscle pain (myalgia). A chest x-ray reveals bilateral infiltrates. You draw a tube of blood from the patient (the tube contains an anticoagulant) and place the tube in a cup of ice. After the blood has cooled, you notice that the red cells have agglutinated (not clotted). This agglutination goes away after you warm up the tube of blood. This patient’s illness is most likely due to infection with which one of the following organisms?

ExplanationAcute interstitial pneumonia refers to inflammation of the interstitium of the lung that is the result of infection, typically with either M pneumoniae or viruses such as influenza A and B. This type of pneumonia is called primary atypical pneumonia because it is atypical when compared to the “typical” bacterial pneumonia, such as produced by S pneumoniae. These bacterial pneumonias are characterized by acute inflammation (neutrophils) within the alveoli. In contrast, acute interstitial pneumonia is characterized by lymphocytes and plasma cells within the interstitium, that is, the alveolar septal walls. Viral cytopathic effects, such as inclusion bodies or multinucleated giant cells, may be seen histologically with certain viral infections. Certain viruses produce pneumonia in certain patient groups, for example, respiratory syncytial virus in infants and adenovirus in military recruits. Infection with M pneumoniae results in the production of a nonspecific cold IgM antibody, which characteristically reacts with red cells having the I antigen. Because most adult red cells have I antigens, blood from a patient with mycoplasma pneumonia will hemagglutinate when cooled. This type of reaction is not seen with infection by P pneumoniae.