Prior to the beginning of the twentieth century transfusion was a dangerous procedure, usually being followed by violent and often fatal reactions. In 1900 Landsteiner1 discovered the cause for these reactions, namely, the presence in the human serum of agglutinins which when mixed with sensitive cells cause marked agglutination. These are called iso-agglutinins because of their specific action on cells from the blood of a subject of the same species and must be differentiated from hetero-agglutinins, which produce similar reactions between the bloods of subjects of different species. On the basis of these iso-agglutinins the four blood groups are distinguished.
Certain errors in determining blood groups occasionally have arisen because of pseudo-agglutination or excessive rouleaux formation (Shattock,2 1900). This condition is common in cases of acute infection and is apparently due to increased viscosity of the serum. Marked rouleaux formation may cause clumping which simulates true agglutination. Slight