In histology, cryptitis refers to inflammation of an intestinal crypt.

Cryptitis is a non-specific histopathologic finding that is seen in several conditions, e.g. inflammatory bowel disease, diverticular disease, radiation colitis, infectious colitis.

As ulcerative colitis is believed to have a systemic (i.e., autoimmune) origin, patients may present with comorbidities leading to symptoms and complications outside the colon. The frequency of such extraintestinal manifestations has been reported as anywhere between 6 and 47 percent, and include the following:

- Aphthous ulcer of the mouth

- Ophthalmic

- Iritis or uveitis, which is inflammation of the eye's iris

- Episcleritis

- Musculoskeletal:

- Seronegative arthritis, which can be a large-joint oligoarthritis (affecting one or two joints), or may affect many small joints of the hands and feet

- Ankylosing spondylitis, arthritis of the spine

- Sacroiliitis, arthritis of the lower spine

- Cutaneous (related to the skin):

- Erythema nodosum, which is a panniculitis, or inflammation of subcutaneous tissue involving the lower extremities

- Pyoderma gangrenosum, which is a painful ulcerating lesion involving the skin

- Deep venous thrombosis and pulmonary embolism

- Autoimmune hemolytic anemia

- Clubbing, a deformity of the ends of the fingers.

- Primary sclerosing cholangitis, a distinct disease that causes inflammation of the bile ducts

In addition to the extent of involvement, people may also be characterized by the severity of their disease.

- "Mild disease" correlates with fewer than four stools daily, with or without blood, no systemic signs of toxicity, and a normal erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP). Mild abdominal pain or cramping may occur. Patients may believe they are constipated when in fact they are experiencing tenesmus, which is a constant feeling of the need to empty the bowel accompanied by involuntary straining efforts, pain, and cramping with little or no fecal output. Rectal pain is uncommon.

- "Moderate disease" correlates with more than four stools daily, but with minimal signs of toxicity. Patients may display anemia (not requiring transfusions), moderate abdominal pain, and low grade fever, .

- "Severe disease", correlates with more than six bloody stools a day or observable massive and significant bloody bowel movement, and evidence of toxicity as demonstrated by fever, tachycardia, anemia or an elevated ESR or CRP.

- "Fulminant disease" correlates with more than ten bowel movements daily, continuous bleeding, toxicity, abdominal tenderness and distension, blood transfusion requirement and colonic dilation (expansion). Patients in this category may have inflammation extending beyond just the mucosal layer, causing impaired colonic motility and leading to toxic megacolon. If the serous membrane is involved, a colonic perforation may ensue. Unless treated, the fulminant disease will soon lead to death.