Leon Lattes developed a method of blood testing that determines the type and characteristics of a dried bloodstain. Bloodstain examinations are often used to gather important forensic evidence in criminal cases.
The four basic blood groups were discovered in 1901. Lattes expounded upon this discovery, making the A-B-O system of blood typing useful in forensics. Until this time, investigators relied on fingerprints to identify suspects.
Blood types are determined by the existence of specific antigens on the surface of red blood cells. Antiserums were developed to pinpoint particular blood types. Blood type A contains antigens that recognize antiserum A. Blood type B contains antigens that recognize antiserum B. In the presence of the matching antiserum, each blood sample becomes clotted. Blood type AB clots when both of these antiserums are added. Blood type O reacts with neither antiserum A nor antiserum B. Differences in blood type were not useful in forensic analysis until this method was discovered by Lattes in 1915.
Dried blood can be more difficult to analyze than wet blood. Certain tests, such as those used to determine drug or alcohol levels, can only be performed on wet blood. Blood dries within 3 to 5 minutes of being exposed to air. As blood dries, its color changes from red to brown.