The present invention provides a method of diagnosing ulcerative colitis (UC) in a subject suspected of having inflammatory bowel disease by by obtaining a sample from the individual; contacting the sample with human histone H1, or pANCA-reactive fragment thereof, under conditions suitable to form a complex of human histone H1, or pANCA-reactive fragment thereof, and antibody to human histone H1; and detecting the presence or absence of the complex, where the presence of the complex indicates that the individual has UC. The invention also provides related methods of diagnosing a perinuclear anti-neutrophil cytoplasmic antibody positive (pANCA-positive) clinical subtype of UC in a patient with UC. In addition, the invention further provides methods of determining susceptibility to UC in an individual. The invention further provides methods of inducing tolerance in a pANCA-positive patient with UC by administering an effective dose of histone H1, or tolerogenic fragment thereof, to the pANCA-positive patient with UC. Compositions of histone H1, or fragment thereof, combined with a tolerogizing molecule also are provided.