The mosquito doesn't, you do.
It's part of the histamine response that also makes it itch.
(The permeability of your capillaries lowers, allowing fluid to accumulate in the intracellular space.)
A bump is a lump like, for example, a bump on your skin or a bump in a road.
1 pair
Bruises or swollen.
keloid...
wart
Wart
Cyst
wart ormilla
lipoma
A painful bump on your skin, usually worse than a bruise.
They do get into fights with other whales or even Giant squid. Also, whales are very big so they bump into things like ship's propellers, which may leave them permanently disabled.
it may be a serois medical conditiion, ask a vet. is the rat old? is the bump big? is the lump swollen? is the bump on a patch of thin skin? where has your rat been? is the bump smooth or scaly or something else?