In 1928, Todd demonstrated the production by group A streptococci of a lysine from human red blood cells. He demonstrated its antigenic character, which causes the production of circulating antistreptolysin O (ASO) antibodies after infection by the microorganism. Streptolysin O is a soluble protein of 69 kDa that binds to the erythrocyte membrane and permeabilises it. The assay of antistreptolysin O enables monitoring the progression of secondary complications related to this condition, which can be acute rheumatic fever or acute glomerulonephritis.
