Hybridogenesis, Resistance to Disease, Blood Types, Xenotransplantation . . .

[vicky_molokh]

Greetings, all!

TLDR: I'm trying to figure out some biomedical implications for some of the weirder species (and their biological interaction with other species) in a setting I'm currently GMing; requesting help.

Basic setup:
Gross biology in the setting is largely similar to that of our world. However, since DNA hasn't been discovered yet, I have the option of declaring that heredity works by mechanisms different than DNA if I need to. There's some measure of mystical/pseudoscientific stuff that may touch upon biology, but delving into it is not necessary so far.

Species group H0 is for most intents and purposes similar to humans; they have some minor variation in neurology, metabolism and eyelid anatomy, but that's about it.

Species S1 is superficially similar to H0, but has differences on a deeper level. Notably, it reproduces strictly through a variant of hybridogenesis with H0, with the latter carrying the offspring (always). S1 have minor differences in metabolism, and a slightly smarter/tougher immune system (it needs to be smarter so as not to cause clashes with the H0's mother's organism during gestation); some of their tissues have biochemical mimicry capabilities for the same reason. S1 tissues are also somewhat less prone to having problems in cases of chimerism (again, 'smarter' matters of compatibility).

Species S2 is a close relative of S1. They have lots of weirder features (including a much greater phenotypal variation amongst themselves), they can gestate their own young, but they also retain the (now strictly optional) hybridogenetic mechanism. They have usually Unusual Biochemistry by our/H0 standards, Resistant to Metabolic Hazards (including an even stronger immune system, albeit one still capable of adapting to contact with H0 tissues).

Further considerations:
I know about the existence of the placental and blood-brain barrier. I tried reading on things like transplant rejection to better understand tissue [in]compatibility, but apparently my biomedical education is insufficient for fully making sense of it. I know that immune systems can be 'trained' in childhood, and heard that 'mistrained' and 'untrained' immune systems are what can result in autoimmune disorders and allergies.

I think that some sort of internal trigger that switches the immune system into learning/adaptive mode from its usual aggressive mode is the way to go for the implementation of such 'racial' abilities, but maybe I'm wrong.
I saw the xenotransplantation section in GURPS Bio-Tech, but it doesn't seem to go into the issue too deep.

Things I wonder / have questions about:
What factors did I miss / should I be aware of when trying to come up with explanations for how this works?
Is it likely that S1 and S2 should have something resembling a Group O Rh- blood type, or would it be the opposite as some sort of highly aggressive/incompatible type?
Does it make sense that it should be possible to enable safe permanent S1->H0 or perhaps even S2->H0 transplants by triggering the 'adaptation mode' of tissues for a while? I'm assuming that taking tissues and organs from an individual with Unusual Biochemistry is a big no-no, but what about the ones who have only RtMH without the former traits?
Just how possible is it to partially or even fully replace an immune system using xenotransplantational methods (bone marrow etc.)?
What other related matters may be a source of something interesting for a campaign where biotechnology plays a role?

Thanks in advance!