Dr. Thomas Dawson wrote a good description of Dandruff in the Hair Transplant Forum May/June 2017 issue as follows:
WHAT IS DANDRUFF?
Normal scalp has few flakes and healthy appearing, smooth skin. Dandruff is characterized by loosely adherent flakes and itching. Specifically, dandruff has no inflammation and is confined to the scalp, while seborrheic dermatitis has yellowish, piled adherent flakes that can extend beyond the scalp to beard, moustache, and eyebrows accompanied by visible inflammation.1 Seborrheic dermatitis is more than superficial flaking, including hyperproliferation, excess lipids, and poorly differentiated corneocytes across the scalp.4 Dandruff requires three factors: fungi (Malassezia), sebum, and individual susceptibility. While there remains much debate in the current medical literature, upon review, the vast majority of evidence points at the very least to a fungal “initiator,” most likely Malassezia fungi.