Translationally controlled tumor protein (TCTP), also called histamine releasing factor, is an evolutionarily conserved multifunctional protein in eukaryotes. We previously reported that extracellular TCTP acquires its cytokine-like function following dimerization. This study aims to identify the functional domain involved in the cytokine-like function of dimerized TCTP (dTCTP). We performed X-ray crystallographic studies and a deletion mutant of dTCTP which lacks the flexible loop domain. Synthetic peptides corresponding to TCTP domains and antibodies developed against them were examined for the anti-allergic effect. In an OVA-induced airway inflammation mouse model, inhibitory effect of synthetic peptides was evaluated. dTCTP was mediated by dimers between Cys172s of TCTP monomers. Synthetic peptides corresponding to the flexible loop and helix 2 domain of TCTP, and antibodies against them inhibited dTCTP-induced IL-8 release. In particular, the TCTP mutant lacking the flexible loop domain decreased the inflammatory cytokine activity of dTCTP. We conclude that the flexible loop and helix 2 domain of TCTP are the functional domains of dTCTP. They may have the potential to be therapeutic targets in the suppression of allergic reactions induced by dTCTP.