Mycoplasma Pneumonia of swine (MPS) decreases the daily growth of pigs, and, co-infection with a virus sometimes causes severe pneumonia. Genetic selection of pigs resistant to the pulmonary MPS lesion might solve the economic loss due to MPS in animal production. Here, we examined the immunophenotype of Landrace line (Miyagino L2), genetically selected to reduce the incidence of pulmonary MPS lesion for 5 generations in Miyagi Prefecture Animal Industry Experiment Station. Although this line is expected to be resistant to the pulmonary MPS lesion, the biological characteristics of its immune function are not clear. We investigated details of the immunorelated phenotype of Miyagino L2 at the hematological and molecular biological level, including cytokine expression, and compared the results with that of non-genetically selected Landrace. Miyagino L2 showed decreased antigen-specific IgG and IgM production and increased CD8-positive T-cell population, and high levels of cortisol concentration, suggesting that the MPS-resistant phenotype is associated these immunological differences. Additionally, T-cell CD4 expression was highly correlated with the MPS expected breeding value. Although the detailed mechanisms underlying this high correlation remain unknown, our result suggested that the genetic selection of the expression level of CD4 might be useful to improve MPS resistance in pig production.