In this episode, Prof Chapman unpacks the science behind immune add-ons in IVF, explaining why an embryo behaves almost like a transplant and how a woman’s body has to tolerate a pregnancy that is half foreign. Responding to a listener’s question about steroids, NK cells and CRP, he separates the one immunological condition backed by real evidence, antiphospholipid syndrome, from the many popular tests that offer little scientific justification. With characteristic warmth and candour, he explains why he remains sceptical of steroids for most patients, what the soon to be released Australian IVF data shows about persistence, and why pregnancy rates keep climbing cycle after cycle even without any add-ons.