Some male anglerfish fuse to the much larger females to mate – they can do so because they don’t recognise their partner’s tissue as foreign material that should be attacked
Source:
https://www.newscientist.com/article/2250429-deep-sea-anglerfish-fuse-bodies-to-mate-thanks-to-an-odd-immune-system/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home