the giving of blood
Raktadaanरक्तदान
The modern daan that most literally gives of oneself.
Raktadaan is the donation of blood. It is the form of daan that most literally gives of the giver: nothing is bought, nothing is manufactured, and there is no substitute for it anywhere in medicine.
Blood also expires. Red cells last a matter of weeks and platelets only a few days, which means a blood bank cannot stockpile its way to safety — it depends entirely on people choosing to give before anyone knows who will need it. India's need runs to millions of units a year, and shortages are routine rather than exceptional.
It fits the classical definition of daan unusually well. The donor never learns who received it, and the recipient never learns who gave — which is close to gupt daan, giving without the possibility of thanks.