Dermatographia is a condition in which lightly scratching your skin causes raised, red lines where you've scratched. The skin cells are overly sensitive to minor injury, such as scratching. This could be the reason why they were hesitant to approve your donation. There are potential risks and complications with plasma donation. Insertion of a rather large intravenous catheter can lead to bleeding, lung puncture (depending on the site of catheter insertion), and, if the catheter is left in too long, it can get infected. It is always good to help others as long as this will not harm you.
Take care and regards.