Antibodies against the virus are quickly raised (about 1 week post infection). These early antibodies are mainly directed against the viral nucleocapsid the immunodominant structural protein but are devoid of neutralizing capabilities.
Neutralizing antibodies are mainly directed against GP3, GP4 and GP5 and eventually GP2 appears late in infection.
The role of neutralizing antibodies is unclear. While some reports showed that passive transfer of neutralizing antibodies are effective in preventing the development of viraemia upon challenge in both sows and piglets if antibody levels reach a critical threshold, it is common to observe the end of the viraemia in absence of detectable neutralizing antibodies. As some authors have reported some strains induce very low or no levels of neutralizing antibodies. Moreover, a recent paper shows that different genotype 1 PRRSV differ in their susceptibility to neutralization and those differences were not explained by the predicted characteristics of GP3, GP4 or GP5, the proteins thought to contain some of the neutralization epitopes. At present, an overall explanation on this unusual antibody response is absent.
Cell-mediated response against PRRSV
The cell-mediated immune response develops gradually and may suffer highs and lows for several weeks or months before showing a plateau.
Several studies have found a qualitative correlation between the level amount of IFN-γ and protection. Besides this, the virus may persist for a prolonged period in the lymphoid tissues of infected pigs. However, this persistence is not lifelong, indicating that the immune system is finally able to clear the virus. Several immune evasion mechanisms have been proposed to be involved.