[...] Blessed Virgin who was immune from [...] actual sin.
Poor choice of words. The BVM wasn't immune from actual sin. She was free from actual sin due to her FREE WILL, not because of a predisposed immunity that protected her from sin. To teach she was immune to sin is to dishonor her, because it says she didn't freely choose Our Lord over sin. She was no more immune to sin than Eve was immune to it. The BVM chose Our Lord, but Eve chose sin.
Accordingly we must hold that she (BVM) was conceived with original sin, but was cleansed from it in some special way.
Another ridiculous comment. If she was conceived with original sin, then she was, de facto, not conceived immaculately. This contradicts the OP's notion that Aquinas taught that the BVM is the Immaculate Conception.