Immutable class vs Immutable struct

I have a class that started life being mutable, but I've since made it immutable. Should I change it to be a struct? What sort of considerations go into choosing one over the other? My particular case is a Point type class (it represents a coordinate in a custom coordinate system) that is made up of 4 int fields, plus a few properties to access the same data in different ways. I note that String is a class and is immutable, so there must be some use-case for this.