CD4 and immune system disorder: In the present study, we demonstrated that long-term intake of heat-killed L. paracasei KW3110 in aged mice significantly enhanced the population of beneficial gut bacteria, of the Bifidobacterium family, and slowed the age-related immune dysfunctions, expansion of the inflammatory IFN-γ-producing CD4-positive T cells in SI-LP, and lowered the serum levels of proinflammatory cytokines.