Porcine circovirus type 2 (PCV2) is recognised as the essential infectious agent of postweaning multisystemic wasting syndrome (PMWS). Although lymphocyte depletion and mon?cytic cell infiltration are characteristic features in PMWS-affected pigs, the mechanism for this remains to be elucidated. This thesis examined the interactions ofPCV2 with T lymphocytes and macrophages. Three immunodominant T lymphocyte epitopes were identified using lymphocyte proliferation assays and synthesised PCV2 peptides. However, the epitopes did not appear to be promiscuous suggesting that a single peptide vaccine for PCV2 infection may not be 100% efficacious. .: Porcine fibrocytes were identified as feasible target cells for a PCV2-specific cytotoxic T .lymphocyte (CTL) assay. However, an apparent PCV2-induced reduction in the population of CD25+ (lL-2Ra) T lymphocytes prevented completion of the assay and may indicate PCV2- induced interference with T lymphocyte proliferation. This appeared to be dependent on the PCV2 genogroup (SGI and SG3) suggesting potential differences in virulence. Pulmonary macrophage studies revealed that the proportion of cells containing PCV2 antigen appeared to be dependent on the PCV2 genogroup which may also contribute to potential differences in virulence. Potential detection of nuclear PCV2 antigen in a small proportion of cells may indicate a macrophage subset that is susceptible to PCV2 replication. Macrophage activation with lipopolysaccharide did not affect the rate ofnuclear PCV2 antigen detection. Cytokine profiling studies found that the only significant differences in the experimentally-induced, PMWS-affected piglets compared to the clinically healthy piglets were reduced IL-2 mRNA levels, increased C-reactive protein levels and increased IL-IO protein or mRNA levels. This may suggest an inappropriate immune response in PMWS-affected piglets. However, no differences were detected in piglets with naturally-acquired PMWS compared to clinically healthy piglets. These conflicting results may be attributable to other factors that influence .the porcine immune response 'including co-infecting pathogens, vaccinations, diet, environmental factors and genetics.