Provided are a composition for inhibiting immune cell proliferation, including sialyllactose or a derivative thereof as an active ingredient, and a method of inhibiting immune cell proliferation, wherein the composition and the method may decrease expression of chemokines, decrease expression of pro-inflammatory cytokines, decrease production of inflammatory mediators, decrease expression of COX2, and decrease production of PEG2, and therefore, may be useful for the prevention or treatment of atopic dermatitis or arthritis.