Question

Case study#2 Assume that you work in criminal investigation unit and were called for an incident that involved someone who was shot. You arrived to the scene to see a dead person in his 50s and no other evidences rather than a small piece of clothing with blood on it. You collected the blood and went to the lab. The Investigation lead to four suspects that happened to be either close to the place of incident or had problems with the victim. You checked the blood type of that blood and it was (O). When you did blood typing for the four suspects, the results came out as the followings: jacks blood reacted only to A, Sams blood reacted to both A and B, Nathans blood reacted only to B, and Johns blood didnt react to either A or B. Which one of the four is the person of interest (suspect) based on the blood typing reaction?

Answer 1

According to the Landsteiner's system of human blood types, there are four major types of blood groups, A, B, AB and O. According to the biochemical compatibilities of these blood groups, only similar type of blood groups can be physically mixed with each other and mixing of opposite or non-similar type of blood groups fail to give the comptability reactions. However, the blood group AB can be mixed with every type of blood group and is physically compatible with all types.

According to the information, the blood type on the cloth was type O. Hence, it is expected to react with none of the blood types since it carries none of the antigens. This can be possible only when the culprit's blood group too is type O. Hence, the information suggests that John's blood did not react with either A or B and hence, it is expected that his blood is type O. This suggests that John must be the suspect.