This study by William S. Tillett and Thomas Francis, Jr. investigates the serological reactions of sera from patients with pneumonia to a non-protein somatic fraction of pneumococci, designated as Fraction C. Fraction C is a carbohydrate derived from the body of the pneumococcus cell, distinct from both the type-specific capsular polysaccharide and the non-type-specific somatic nucleoprotein. The authors found that sera from patients with lobar pneumonia, especially during the acute phase, exhibited high titers of precipitins for Fraction C, which disappeared within 1 to 3 days after the crisis. This reactivity was not observed in normal individuals or sera from patients with other febrile diseases, suggesting a specific association with pneumococcal infection. The study also noted that the appearance of precipitins for Fraction C was early in the disease and could be related to previous exposure to bacterial antigens. Additionally, the presence of Fraction C precipitins was more common in cases where Gram-positive cocci were identified as the etiological agent. These findings highlight the complexity of the immune response to bacterial infections and suggest a potential broad relationship among certain bacterial infections.This study by William S. Tillett and Thomas Francis, Jr. investigates the serological reactions of sera from patients with pneumonia to a non-protein somatic fraction of pneumococci, designated as Fraction C. Fraction C is a carbohydrate derived from the body of the pneumococcus cell, distinct from both the type-specific capsular polysaccharide and the non-type-specific somatic nucleoprotein. The authors found that sera from patients with lobar pneumonia, especially during the acute phase, exhibited high titers of precipitins for Fraction C, which disappeared within 1 to 3 days after the crisis. This reactivity was not observed in normal individuals or sera from patients with other febrile diseases, suggesting a specific association with pneumococcal infection. The study also noted that the appearance of precipitins for Fraction C was early in the disease and could be related to previous exposure to bacterial antigens. Additionally, the presence of Fraction C precipitins was more common in cases where Gram-positive cocci were identified as the etiological agent. These findings highlight the complexity of the immune response to bacterial infections and suggest a potential broad relationship among certain bacterial infections.