Generally, white blood cells do not mix during sex. However, it is possible but only if both people involved share blood, which sometimes happens if tissue is cut or abraded during intercourse. Mixing blood can be very dangerous, which is why doctors are so careful when giving blood transfusions. Nowadays, a donor’s blood is separated into its parts (red blood cells, white blood cells, and platelets) before it is transfused and the white blood cells are completely removed. Receiving the wrong type of blood is very rare nowadays, but it can happen. Interestingly, the first symptom patients usually report is a literal “feeling of impending doom,” or a feeling that something really bad is about to happen. Other warning signs are fever, aches, chills, and a burning sensation at the injection site. Luckily, this is very rare in today’s medical world!