CD4 and infection: The lack of a substantial FLUAVsw infection-related expansion of CD4+ T cells (in all animals) might be also due to a masking by T-cell responses to other environmental antigens encountered following weaning, and by T-cell responses to the vaccines (porcine circovirus type 2, Mycoplasma hyopneumoniae, see chapter 2.1) administered at three weeks of age.