This combination of cytokines also stimulates IFN-gamma production. The cell-mediated response is the most important contribution of the immune system in fighting toxoplasmosis. The main value of the humoral response seems to be opsonization. The complement pathway and NK cell activity are both increased if parasite cells have antibody bound on their surface. Resistance to T. gondii can either be natural or acquired.
Natural resistance is based in different genetic traits.
For example, different species and strains exhibits higher and lower susceptibilities. For example, rats, guinea pigs, and non-human primates are all at least somewhat resistant to T. gondii . There are two main causes for acquired resistance. First, infection with an avirulent strain can provide resistance without causing symptoms. Secondly, an animal infected with a virulent strain will show resistance to reinfection.