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Say there is a plant-based toxin that causes what is essentially a state of deep sedation but has no other real effects. In this world there is one animal that is immune to this toxin (the plant is part of it's diet). There are hybrid creatures that, if close enough genetically to that particular animal can be affected by this toxin but only temporarily. Any other animal exposed, human or otherwise would usually be affected permanently.

Now say a person developed immunity to this toxin through mithridastism (whether knowingly or unknowingly being dosed with tiny amounts of the toxin over time). A couple of questions.

  1. If this person was a woman and she was pregnant during the time (or part thereof) that she was dosed with this toxin, could the child born also be immune?

  2. Could the blood of the child or the mother be used to create an anti-toxin in the same way the blood of non-human animals is used to create real life anti-toxins, and if so is there a time restraint on how long after exposure to the toxin this could be done (as with creating real life anti-toxins the blood is usually taken after a period of weeks or months), both in terms of blood taken from the immune person and anti-toxin administered to the affected?

Basically, would it be possible for an anti-toxin to be 'stumbled upon' in this way if previous research into treatment for those affected was intentionally sabotaged and it is widely believed there is no possible cure (as it seems pretty obvious someone would have at least tried to create an anti-toxin or something in the past given that they know there are creatures with full or partial immunity).

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In my opinion, your premise is within realm of plausibility.

As wiki notes, Mithridatism is most likely to work against biological poisons that can be recognised by immune system resulting in immune system creating antibodies to bind and inactivate poison molecules before they can do harm.

You might have to drop "permanent" part of sedation for that to work. Usually anaesthetics and nerve-oriented poisons work by binding to receptors in nervous cells preventing propagation of signal, or preventing deactivation of once initiated signal. If unable to kill, eventually such chemicals become metabolised and once blocked pathways and receptors are cleared to function again (high but non-lethal concentrations can cause permanent damage in other ways). If poison is never metabolised then small incremental doses will keep building up since every molecule not intercepted by antibodies will disrupt one nerve cell forever, increasingly disrupting consciousness and bodily functions (think being intoxicated, not narcolepsy) until reaching lethal levels of disruption or rendering person/animal a vegetable.

One animal being able to ignore your plant's poison does not at all imply possibility of humans building up resistance. Moongooses (Mongeese?) are unaffected by snake's neurotoxin because of unique shape of receptors to which $\alpha$-neurotoxin is supposed to bind leaving them unable to operate; snake's neurotoxin can not attach itself to those receptors. It's a specific adaptation to hunting snakes (snakes also have unusual shape of those receptors, just to be clear, so snakes are also unaffected). You have no way of transferring modified neuroreceptors to humans, outside of slow mode called evolution (unless your world is in beyond ours genetic engineering phase) which could lead to so called convergent evolution giving some humans complete, inheritable immunity.

According to research dating back to 80s newborn's immune response is "immature" not due to lack of antibodies, but due to lack of antigen (think poison or pathogen, antigens are things that antibodies bind to to tag or neutralise them) stimulation. Summary of paper also states that late-term foetus and newborn baby don't rely exclusively on antibodies "given" by mother, newborn's blood contains antibodies designed to fight mother's lymphocytes, remember that cells themselves are not supposed to pass placental barrier between mother and fetus so ability to secrete such antibodies should be beneficial. Think of immune system not knowing what exactly to fight. Diseases and poisons can hinder development of fetus in many different ways so mother's body becomes extremely zealous about fighting anything that could threaten child-in-making. As such, it is plausible that fetus exposed to your biological poison (poison would have to pass from mother through placental barrier) could start "training" resistance to it, assuming that poison doesn't kill it. That still leaves risk of severe disorders (especially in case of poisons that take very long time to metabolise). Teratology has a long list of disorders caused by toxicants (Drugs in pregnancy, Environmental toxicants and fetal development) ranging from low birth mass through cognitive impairment to death. Famous case was Thalidomide which caused children to be born without limbs, if drug (as in drug the medicine, Thalidomide is used in treating some types of cancer) was taken in early stage of pregnancy.

Extracting antibodies out of blood to create antivenom would be possible, however it's a pretty advanced science (very late XIX century to very early XX century) so plausibility of this part depends on details of your world. Note however that such antivenom will do nothing to help someone already affected, it can only help prevent sedation or death if given fast enough after exposure to poison.

Summary and suggestions

Now, where do we go with this? As I said, fetus building up resistance if mother is poisoned with this poison and poison can pass placental barrier is plausible. This is however immunity in medical sense of being able to resist poison, not immunity in common sense of being unaffected, that requires having different structure of cellular receptors like above mentioned Mongoose. Depending on your world building, you can use sideeffects of prenatal immunisation to include some bonus flavour (or work towards some social/political statement). For example you can create society which is routinely exposed to your poisonous plant and all members of said society are highly resistant but suffer some disorder (your choice, you can make it almost anything, from unusual eye colour though increased paranoia to missing right arm), for ancient-to-feudal-level world you can have nobility valuing resistance to poisons but suffering from, say, increased aggressiveness or diminished intelligence, you can have families of people working with this plant (because it has some other economically important characteristic, e.g. makes good fabrics) who are stigmatised due to abnormally long arms.

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Now say a person developed immunity to this toxin through mithridastism

This sort of immunity isn't a permanent immunity, and will diminish over time. In order to maintain it, you've got to keep eating snoozyherb forever, in the form of some reasonable dose over some reasonable time period. Wait long enough and those antibodies will be flushed out of your system, and production may not ramp up fast enough to save you next time you encounter an effective dose of the offending chemical.

If this person was a woman and she was pregnant during the time (or part thereof) that she was dosed with this toxin, could the child born also be immune?

Antibodies are passed to the baby in the third trimester, and some are passed on during breastfeeding, so the baby may initially have some resistance to sleepyweed by this will wear off in relatively short order. That's why real-world infant immunisation starts in the first few months of life.

Again, the sort of resistance given by mithridatism isn't permanent, and as it isn't genetic it won't be heritable.

Could the blood of the child or the mother be used to create an anti-toxin

The mother, potentially. Obviously, she's only one human, so there's only so much blood you can suck out of her body in some time period if you don't want to cause unfortunate side effects like death. The child would carry the antibodies too, but they're kinda small and so there's even less you could extract from them.

Real-world antivenom manufacture is done in bulk, using multiple animals. Perhaps the existence of one immune individual would lead to the discovery of how the immunity was acquired and production in larger volumes (possibly using animals rather than people) would be done instead.

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Babies get their antibodies from their mother in the womb. Out of the womb the baby's immune system has to fend for itself to become immune. The mother will add antibodies to the mothers milk to help protect the child from whatever diseases the mother is exposed too (one reason why 100% of mothers kiss the baby's cheeks, they take a sample of the diseases on the child to create antibodies for). But this does not make the child immune, nor does the blood itself contain immunity.

Reading this article: https://www.nature.com/news/eye-opening-picture-of-fetal-immune-system-emerges-1.22144 it seems that fetal immune systems are far more active in the early stages, but since the baby still develops a wide range of "standard" diseases after being born and the baby does not inherit immunity that the mother has (like being innoculated for the mumps before pregnancy does not make the child not need innoculation but does offer him motherly antibodies) it is still required that the baby goes through immunization itself.

But we are even talking about something a bit different: immunity to toxins. This can be solved in at least two ways: something is released in the bloodstream to absorb/bind the toxins and make them harmless or the cells themselves find ways to deal with the toxin before it does any harm, for example by developing a mechanism that prevents toxins from being absorbed in the first place. Since the baby's immune system is already not up to specs to deal with normal diseases it is unlikely it'll have the necessary capabilities to deal with toxins and will have to rely on the mother to protect it.

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