1. A biosensor, comprising: a photoplethysmography (PPG) circuit configured to obtain a plurality of spectral responses at a plurality of wavelengths detected from skin of a user; a processing circuit configured to: determine a first ratio R value using a first spectral response and a second spectral response of the plurality of spectral responses, wherein the first ratio R value varies based on one or more of a plurality of types of antigens on surfaces of red blood cells in a blood stream of the user; access a calibration database that associates predetermined ratio R values to the plurality of types of antigens on the surfaces of red blood cells; and identify a blood type of the user using the first ratio R value and the calibration database.