Upgrds on Instagram: "Blood types are determined by tiny molecules called antigens that sit on the surface of red blood cells. Think of them as ID tags. Your immune system uses these markers to decide which cells belong in your body, and which don’t. The main classification is the ABO system: Type A has A antigens Type B has B antigens Type AB has both Type O has neither There’s also the Rh factor, which indicates whether a specific protein is present (+) or absent (–). So: A+ has A antigens and