Differential PCV2-specific IFN-γ secretion responses upon PCV2 natural exposure. Representative responses of one PCV2-vaccinated (A) and one unvaccinated (B) pig at 19 weeks of age (+4 weeks post-exposure (PE) to PCV2 natural infection) characterized respectively by stable IFN-γ SC number and increased/higher IFN-γ productivity per cell (A) or increased cell number and stable/lower IFN-γ productivity per cell (B) depending on the amount of PCV2b used as ex vivo recall antigen. The numbers are relative to the spots counted in the stimulated well shown (2 × 105 PBMC) and (in brackets) the mean of the corresponding unstimulated negative control wells (2 × 105 PBMC). The spot size and spot intensity distributions are shown on a linear scale. PCV2-vac pigs were vaccinated at 3 weeks of age using a PCV2a Cap protein-based subunit vaccine.