Yes, LYMPHOCYTES is a Scrabble word!

Letter values in Scrabble US: L 1, Y 4, M 3, P 3, H 4, O 1, C 3, Y 4, T 1, E 1, S 1.

Is it a valid word?

LYMPHOCYTES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

an agranulocytic leukocyte that normally makes up a quarter of the white blood cell count but increases in the presence of infection

Also: lymph cell.