** 1. itchy rash could be an allergic reaction to the procedure involving insertion of needle into the vein to extract plasma, and the site from where the blood is taken, may appear a bit swollen and bruised, it is important that you eat well and re-hydrate during this process PS. First-time donors, teenagers, and women are at a higher risk of a reaction. [so if any factor is with you, possibility is likely] 2. quercetin is a plant pigment which acts as an anti histamine and anti inflammatory, and can cause headache and tingling of the arms and legs 3. Initially seek help of doctor who can give you medicines to help stop the reaction.