I'm not sure whether this question is redundant, but I think it is probably not due to the focus of the question.
We might say that photon does have antiparticle and it is photon it self. I know for a charged particle, it must have an antiparticle due to the constraint imposed by quantum field theory. However, photon is neutral, I want to know whether there is any constraint imposed by any theory that photon MUST have an antiparticle, even though it is just photon itself?
Therefore, what I want to know is that is there any constraint, and then where does it come from. In addition, I want to know whether the statement "every particle in the Standard Model has an antiparticle" is precise or not.