At the most fundamental level, photons can interact directly only with charged particles. From the point of view of Feynman diagrams, the only "vertices" that involve photons have two lines for charged particles and one line for the photon.
However, there are second-order processes in which two photons can interact indirectly, by using virtual charged particles. A Feynman diagram for one such process looks like a square box (for the charged particles) with wavy lines entering two corners and leaving two corners (for the incoming and outgoing photons). This is the sort of thing that Hans is referring to. Since it's a higher-order interaction, its probability is very low which makes it difficult to detect.