This invention provides antibodies that interact with or bind to human B7 related protein-1 (B7RP1) and antibodies that bind to and neutralize the function of B7RP1 thereby. The invention also provides pharmaceutical compositions of said antibodies and methods for neutralizing B7RP1 function, and particularly for treating immune disorders (e.g., inappropriate immune response) by administering a pharmaceutically effective amount of anti-B7RP1 antibodies. Methods of detecting the amount of B7RP1 in a sample using anti-B7RP1 antibodies are also provided.