‘Around the second trimester the fetal immune system becomes mature enough to discriminate self from non-self.’
There’s a metaphor in there somewhere but my brain has grown so accustomed to being rational that I do not remember how to use surrealism. sad face.
Current belief is that discrimination of self/non-self is acquired during the fetal period, with no further development thereafter. Some immunologists question this and experiments with APCs and controlled antigen dosage to convince the adult immune system to accept foreign cells as self show promise. Insert additional metaphors about acquired xenophilia.