Anti-streptolysin O is an antibody produced by certain immune cells when they encounter streptolysin O. The streptolysin O is a form of streptolysin that is oxygen-labile. It is a β-hemolytic enzyme belonging to a family of thiol-activated toxins. It is produced by and isolated from groups A, C and G streptococci. This cytolysin is capable of causing lysis in animal cells, especially WBCs and RBCs. It does so by binding with the membrane cholesterol where it polymerizes to form a transmembrane channel (about 30 nm in diameter). This leads to cell lysis. Thus, streptolysin O is a potent cell poison responsible for the lysis of WBCs and RBCs.
In the presence of this toxin, the immune cells produce antibodies, such as anti-streptolysin O. Because of this, it is useful in determining exposure to streptococci infection. Anti-streptolysin O titer is a measure of the plasma level of such antibodies in the blood. A high titre, e.g. >200 units in adults or that which is considered beyond the acceptable value, could mean a need for an antibiotic treatment, e.g. penicillin.