This invention provides antibodies or functional fragments thereof that bind to PD-1 with high affinity. The invention provides nucleic acid molecules encoding the antibodies or the fragments thereof according to the present invention, expression vectors and host cells for expressing the antibodies or the functional fragments thereof according to the present invention, as well as methods for producing the antibodies or the functional fragments thereof according to the present invention. The present invention also provides immunoconjugates and pharmaceutical compositions comprising the antibodies or the functional fragments thereof according to the present invention. The present invention additionally provides methods for treating a plurality of diseases (comprising cancers, infectious diseases and inflammatory diseases) by using the antibodies or the functional fragments thereof disclosed herein.
