If anyone is immune it would be Judith.An infant's immune system is at it's best in the womb and in the first 2 or so years of life . A baby is pretty resilient and in the Walking Dead world it would make sense that children would begin to get immunity to the virus or whatever microbe is causing all the crapAmy was also bit in the neck/shoulder area and probably had her subclavian artery severed.
I can't see Tyreese being immune.