Many cnidarians, including reef-building corals, establish symbiotic relationships with photosynthetic algae of the family Symbiodiniaceae. Previous studies have implicated host innate immunity as playing a role in the maintenance of cnidarian-algal symbiosis and it has been shown that establishing symbiosis with different genera of algal symbionts modulates thermal tolerance. Using the facultatively symbiotic coral, Oculina arbuscula we investigated how host immunity, in particular NF-KB expression, was modulated in the presence and absence of intracellular algal symbionts. We used menthol to induce symbiont expulsion to generate aposymbiotic and symbiotic fragments of the same coral genet. We then assayed NF-KB protein levels through Western blotting and compared protein levels between symbiotic states. We relate these findings to our prior transcriptomic profiling of symbiotic and aposymbiotic branches of colonies of the same species and contrast patterns across other taxa that host intracellular photosymbionts including the anemone Aiptasia, the sponge Cliona, and the salamander Ambystoma. Our results shed new light on the mechanisms that govern healthy symbiosis with Symbiodiniaceae. As heat-induced bleaching threatens corals across the world, there is an urgent need to understand how these partnerships will respond to changing environmental conditions.