Yes, what is 'healthy' looks different for each individual. That doesn't mean that something is found to be 'healthy' because it is 'self'. The immune systems

Yes, what is 'healthy' looks different for each individual. That doesn't mean that something is found to be 'healthy' because it is 'self'. The immune systems primary function is housekeeping: killing cells that are still completely 'self', but are old, worn down or dysfunctional in some other way. Killing 'invaders' and cancers is minor in terms of the numbers of cells destroyed. As your transplant example shows: something foreign can be found to be entirely 'healthy' by your immune system. So self vs. foreign is too narrow.