IFNG and periodontitis: As for +874A/T, there are 3 possible genotypes: the AA, AT, and TT genotypes, which are thought to represent 3 different expression levels of IFN-γ: low, intermediate, and high, respectively.[26,33] The different level of IFN-γ can affect the immune response and susceptibility to inflammatory diseases.[43,44] Reichert et al[29] concluded that IFN-γ TT genotype could have a lower susceptibility to periodontitis because of the high quantity of cytokine production in this genotype.