Objective       To identify systematically MHC-restricted immunodominant T helper lymphocyte (Th) epitopes from urease subunit B of Helicobacter pylori (H. pylori) and to confirm the MHC-restricting profile.       Methods       BALB/c mice were immunized with recombinant UreB protein (rUreB) and Freund’s adjuvant by subcutaneous injection. Antigen-specific CD4

+

T cells were amplified in vitro from spleens of immunized mice. The MHC-restricted immunodominant Th epi

topes were identified by synthesizing overlapping peptides and antibody-blocking assay.       Results       UreB

403-420

and UreB

409-426

induced the strongest secretion of IFN-γ in all the 93 18mer peptides

while 13mer peptide UreB

403-426

within UreB

403-420

and UreB

409-426

induced equivalent responses compared with the 18mer peptides. The results of antibody-blocking assay showed that the H-2

d

(I-A) monoclonal antibody efficiently blocked the T cell activation induced by peptide UreB

403-426

.       Conclusion       UreB

403-420

and UreB

409-426

are immunodominant Th epitopes of urease subunit B antigen. The core reactive sequence is UreB

409-421

and its restriction molecule is H-2

d

(I-A).