Why one cat bites another cat's neck:
1. Neck biting is a relatively common phenomenon. It may occur during a fight or while playing. It is usually a light bite and will not cause serious injury.
2. If the older cat bites the kitten's neck, it may be that the older cat is accepting the kitten and treating it like its own child.
3. It's possible they are mating. If a male cat is riding on a female cat and biting its neck, it's very likely that the two cats are mating. 
Precautions after a cat bites your neck:
1. Check the cat's neck for any wounds. If there are any, treat them with anti-inflammatory and antibacterial agents promptly.
2. Check if the cat is biting or being bitten on the neck because it is in heat. If the cat is in heat, you can take it to a veterinary hospital for spaying or neutering.
3. If the neck biting is caused by fighting, it should be stopped immediately, the cat isolated, and the cat trained.
