PRP is a concentration derived from your own blood. It has lots of good growth factors that stimulate hair growth when injected into the scalp. PRP may or may not contain red or white blood cells. But, by definition, PRP contains a concentration of platelets that is higher than your blood. Once in the scalp, PRP growth factors begin to degrade quickly so any benefit to thinning hair will be short lived. Therefore, the PRP injection needs to be repeated several times a year to maintain good benefit.