Blood Group Tattoo
Quite common among people who have served in the Armed Forces (Army, Navy or Airforce) is a tattoo of the wearers blood group, such AB or AB negative. Ostensibly used to give a medic quick information in the case of an injury which leads to a Blood Transfusion, it is more likely that the tattoos have a placebo affect of giving the wearer peace of mind. Though it is possible that a tattoo like this has saved a soldiers life.
In nazi Germany, members of the Waffen-SS would have blood group tattoos under their left arm. The tattoo would be around 7mm long and was placed about 20cm above the elbow, consisting of a single letter (A, B or O, there are no known examples of AB blood group tattoos in the Waffen-SS) in either gothic or common latin style lettering. Since the rhesus factor was not properly understood yet, it was not implemented. Right after the end of world war II, the allied forces would sometimes identify members of the Waffen-SS by the blood group tattoo.(Citation needed)