Compositions and methods are disclosed for inhibiting the release of a proinflammatory cytokine from a vertebrate cell, and for inhibiting an inflammatory cytokine cascade in a patient. The compositions comprise, for example, high affinity antibodies that specifically bind HMG1 and antigenic fragments thereof. The high affinity antibodies of the present invention and pharmaceutical compositions comprising the same are useful for many purposes, for example, as therapeutics against a wide range of inflammatory diseases and disorders such as sepsis, rheumatoid arthritis, peritonitis, Crohn s disease, reperfusion injury, septicemia, endotoxic shock, cystic fibrosis, endocarditis, psoriasis, psoriatic arthritis, arthritis, anaphylactic shock, organ ischemia, reperfusion injury, and allograft rejection. In addition, the high affinity antibodies of the present inventions are useful as diagnostic antibodies.