Page "Antiparticle" Paragraph 5

Although particles and their antiparticles have opposite charges, electrically neutral particles need not be identical to their antiparticles.

The neutron, for example, is made out of quarks, the antineutron from antiquarks, and they are distinguishable from one another because neutrons and antineutrons annihilate each other upon contact.

However, other neutral particles are their own antiparticles, such as photons, the hypothetical gravitons, and some WIMPs.