The Humors Reborn: Blood Types and Immune Compatibility

In my Canon of Medicine, I wrote that blood differs between individuals—that constitutional differences determine health and disease. I taught four humors: blood, phlegm, yellow bile, black bile. The mechanism was wrong, yet the principle proved true. Modern medicine vindicates this insight with a precision I could not have imagined: blood comes in distinct types, classified by molecular markers invisible to the eye but decisive for life itself.

Eight Types From Four Humors

I advocated that physicians must know patient constitution before treatment. Bloodletting, I warned, must account for individual differences or risk harm. Now: the ABO system reveals four fundamental types based on antigen presence—A, B, AB, O—glycoprotein molecules embedded in red blood cell membranes. The immune system produces antibodies against antigens absent from one’s own cells. Type A possesses A antigen, produces anti-B antibody attacking foreign B cells. Type B the reverse. Type O has neither antigen but both antibodies—paradoxically universal donor for red cells, yet universal recipient rejected. Type AB has both antigens, no antibodies—universal recipient, limited donor.

The Rh system adds complexity: 61 different antigens, RhD most immunogenic, distinguishing positive from negative. Four ABO types multiplied by two Rh designations yield eight common blood types. Transfusion without matching triggers hemolysis—red cell destruction, potentially fatal. My humoral theory was crude approximation; modern immunology corrects the mechanism while preserving the principle: blood differs fundamentally between individuals, mixing incompatible types causes illness, know patient’s constitution before intervention.

Discovered 1901 by Landsteiner, the ABO system; 1940, the Rh system. Medicine took a millennium to reach the systematic blood classification I advocated in the Canon.

Universal Donor, Universal Need

Type O negative: universal red cell donor, lacking ABO and RhD antigens, cannot trigger immune response in any recipient. Relatively rare—seven percent of population—yet most valuable for emergencies. Trauma, battlefield medicine, situations permitting no delay for typing. During the Second World War: 200,000 pints shipped to European theaters, 50,000 pints for D-Day alone. Modern medicine can identify compatible blood but cannot manufacture it. Still depends on donor generosity as medicine has always depended on healers’ virtue.

In my era, bloodletting often harmed by removing what the body needed. Now scarcity harms by denying beneficial transfusion. Medicine evolved from balance-by-removal to healing-by-addition, yet constraint remains.

Golden Blood: The Quintessence

Rh null—the rarest essence. Lacking all 61 Rh antigens, found in fewer than fifty individuals globally, called golden blood for its universal compatibility with rare Rh-negative variants. This is the quintessence sought by alchemists: purest form, most potent effect, cannot be manufactured, depends on fortunate discovery.

For recipients with unusual Rh combinations missing multiple antigens, Rh null may be the only compatible option. Yet possessing this gift creates burden. Donors face lifetime obligation—on call perpetually, knowing refusal may cost a stranger’s life. A Swiss donor took taxi to Geneva, missed work, absorbed costs without reimbursement. European law prohibits payment for blood donation. Ethical weight of scarcity: at what point does moral obligation to help end when helping requires substantial personal sacrifice?

Medieval alchemists sought philosopher’s stone transmuting base to noble. Modern hematology found golden blood through chance genetic mutation. Neither can be created by will. Both reveal that rarest resources often come with heaviest responsibilities.

Knowledge of anything, I wrote, is not complete unless known by its causes. Blood types reveal hidden constitutional individuality—what my four humors glimpsed darkly, immunology illuminates with precision.