I need a temporary mechanism for blood type switches for my modern love story, the protagonist is a werewolf and would transform every time when there is a full moon. I was thinking maybe a certain type of protein can react to the moonlight at 0.3 lux minimum depending on the location and weather condition etc, I was wondering is it possible for the antibodies in blood to undergo a temporary changes and the side effects are red eyes, sharp teeth and premature blossoming of body hair etc? the DNA is still human but the subject is in a state of delirium akin to lycanthropy where s/he begins to suspect s/he is actually turning into a beast as a result of the unsightly appearance. Is there a similar medical condition that I can borrow from or do I invent one?

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    $\begingroup$ The title and the body appears to be two entirely different questions. Which is it? $\endgroup$
    – user93359
    Jan 4, 2022 at 4:33
  • $\begingroup$ Bit difficult for teeth to sharpen over the course of a single lunar cycle all by themselves, though apparently some sharks can develop new teeth over a timescale of a few weeks. $\endgroup$ Jan 4, 2022 at 14:43

2 Answers 2


Specific side effects no, but there is one medical condition that routinely removes the A and B antigens from human tissue, carcinoma, that could be a useful starting point for a condition as radical as lycanthropy, or not. If you are open to a version of lycanthropy caused by a viral or bacterial parasite this may be of interest as well.

Note: Once stripped of the surface proteins that give blood its ABO type the cells won't be able to regrow them in any way that could be considered a normal biological process/effect. New cells created between expressive episodes would carry their normal protein coating until the next episode though giving the lycanthrope a low but measurable antigen load. Only 1/4 to 1/3 of their cells will have their antigen coating at any given time though, unless they replace red cells faster than a normal human in which case they may be able to have a much higher, more normal, antigen load.

  • $\begingroup$ This is good, but it partly depends on how deep the question needs to be. The front-type (antigens) can change temporarily because of this, but the back-type (antibodies) don't; so a blood bank would give an intermediate typing or weak back-typing when tested. There are some autoimmune diseases that can account for the back-typing, and autoimmune diseases can cause some really strange symptoms at times... $\endgroup$
    – DWKraus
    Jan 4, 2022 at 5:49
  • $\begingroup$ How does carcinoma change blood type? I am skeptical. And "carcinoma"? There are dozens! $\endgroup$
    – Willk
    Jan 4, 2022 at 21:58
  • $\begingroup$ @Willk Have a look at the clinical applications here it doesn't go into the how/why just states that in most humans a major event in the development of any carcinoma is the suppression of the production of A and B antigens in the host. $\endgroup$
    – Ash
    Jan 5, 2022 at 2:43
  • $\begingroup$ @Ash - thanks for the link. It is the carcinoma cells themselves that fail to express the antigens. That makes sense; those cells are defective monsters. $\endgroup$
    – Willk
    Jan 5, 2022 at 14:08
  • $\begingroup$ @Ash if that were the case it would become difficult to ascertain blood type on cancer patients to transfuse them but that does not happen. But loss of antigens (of all types!) from cancer cells themselves definitely happens - both randomly because of their screweduppedness and as a result of selective pressure by the immune system. $\endgroup$
    – Willk
    Jan 6, 2022 at 18:22

Bone marrow transplants can change your blood type.

If I am type AB and I get a bone marrow transplant from a type O donor I will eventually be type O when the cells engraft. Not only that but I will have type O lymphocytes and they do not like AB blood. My new immune system will wipe out any remaining AB blood I have around; I will be jaundiced from the hemolysis as the AB cells are destroyed leaving me with only O.

This can happen with solid organ transplants too as they may come with some blood stem cells.

Changing Blood Types and Other Immunohematologic Surprises

With the advent of transplantation of bone marrow and solid organs, a growing number of unexpected immunohematologic findings have been noted, especially when the donor and recipient are of different ABO blood groups. In this issue of the Journal, Comenzo et al.2 add to the list of anomalies by describing a patient whose blood type transiently changed from group O to group AB after transplantation of a liver from a group AB donor.

When you change back to a type AB you would not get the hemolysis as type O is the universal donor and so the AB lymphocytes won't attack your old type O blood. But you would type as AB even with just a little bit of AB blood.

The idea of different stem cell populations cycling on and off as the basis for lycanthropy is an interesting one. I struggle to think of I biological way it might be triggered by moonlight but it could be triggered by a premenopausal woman's monthly cycle that was aligned with the phase of the moon.


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