Inflammatory bowel disease (IBD) is a group of chronic disorders characterised by abnormal inflammation within the gastrointestinal tract. The two principal forms are Crohn's disease and ulcerative colitis, although microscopic colitis is another recognised form of IBD. IBD is fundamentally different from irritable bowel syndrome (IBS): IBD involves objective inflammation and can cause structural tissue damage, whereas IBS is a disorder of gut–brain interaction without the intestinal destruction characteristic of IBD.
The inflammation in IBD results from inappropriate or dysregulated immune responses interacting with genetic susceptibility, the intestinal microbiome and environmental influences. Disease activity commonly fluctuates between periods of active inflammation, called flares or relapses, and periods of reduced or absent clinical activity, called remission. IBD can begin at any age but frequently starts in adolescence or early adulthood, and its chronic nature means long-term monitoring is often required.
Crohn's disease and ulcerative colitis share some symptoms but differ substantially in anatomical distribution and the pattern of inflammation. Crohn's disease can affect any part of the gastrointestinal tract from the mouth to the anus and commonly affects the terminal ileum and colon. Its inflammation can be patchy, with diseased areas separated by apparently unaffected areas, and it can extend through the full thickness of the bowel wall.
Ulcerative colitis, by contrast, affects the large intestine and begins in the rectum, extending proximally to varying distances through the colon. Its inflammation is primarily confined to the inner mucosal lining. These anatomical differences explain why Crohn's disease can cause strictures, fistulas and abscesses, whereas ulcerative colitis has a different complication profile. Some patients have inflammatory bowel disease that cannot initially be confidently classified as Crohn's disease or ulcerative colitis, and clinicians may use terms such as IBD-unclassified while further evidence is obtained.
The exact cause of IBD is not fully established. Current models describe it as an inappropriate immune response occurring in a genetically susceptible person and influenced by environmental factors and the intestinal microbiome. The immune system normally tolerates the enormous population of microorganisms in the intestine while responding to genuine threats, but in IBD this regulation becomes abnormal, leading to persistent inflammatory signalling.
Genetic variants can alter immune pathways, epithelial barrier function and microbial interactions, but genetics alone cannot explain the disease. The microbiome also differs in many people with IBD, although it is not yet possible to identify one microbial organism as the universal cause. Environmental exposures, diet, smoking, medications and other factors can modify disease risk or activity, with effects differing between Crohn's disease and ulcerative colitis. IBD should therefore not be described as being caused simply by stress, poor diet or a single infection.
Symptoms depend on the location and severity of inflammation. Common gastrointestinal manifestations include persistent or recurrent diarrhoea, abdominal pain, urgency, rectal bleeding and changes in stool frequency. Crohn's disease can additionally cause weight loss, malabsorption, abdominal cramping and nutritional deficiencies, particularly when the small intestine is affected. Ulcerative colitis commonly produces diarrhoea containing blood, urgency and abdominal discomfort because inflammation often involves the rectum and colon. Systemic manifestations can include fatigue, fever and anaemia.
IBD can also affect organs outside the digestive tract. Inflammatory complications may involve the joints, skin, eyes, liver and bile ducts, and kidneys in some patients. These extraintestinal manifestations arise because IBD is a systemic immune-mediated disease rather than merely a local bowel disorder. Their presence can sometimes help clinicians recognise the broader inflammatory nature of the illness.
Diagnosis of IBD normally requires integration of clinical history, examination, laboratory testing, stool assessment and direct evaluation of the bowel. Blood tests can identify anaemia, inflammatory changes or nutritional deficiencies, while stool tests can help identify infection and can measure inflammatory markers such as faecal calprotectin in appropriate circumstances. Colonoscopy with biopsies is particularly important for assessing ulcerative colitis and colonic Crohn's disease because it permits direct visualisation and microscopic examination of the intestinal lining.
Imaging such as magnetic resonance enterography, CT, ultrasound or other specialised techniques can be useful for assessing small-bowel disease and complications that cannot be adequately evaluated by colonoscopy. No single test establishes every case. The diagnosis depends on the pattern of inflammation, anatomical distribution, histological findings and exclusion of alternative explanations. This contrasts with IBS, which is usually diagnosed through characteristic symptoms and appropriate exclusion of important alternative disease.
Treatment aims not merely to suppress symptoms but to control intestinal inflammation, achieve remission and prevent complications and irreversible bowel damage. Depending on disease type, location and severity, treatments can include aminosalicylates, corticosteroids, immunomodulators, biologic medicines and newer targeted small-molecule therapies. Corticosteroids are generally used to induce control of active inflammation rather than as indefinite maintenance treatment because prolonged exposure causes substantial adverse effects.
Biologic and targeted therapies act on specific immune pathways and can be used to induce and maintain remission in appropriate patients. Nutritional support can be important, particularly in Crohn's disease with malabsorption or weight loss. Surgery has an important but different role in Crohn's disease and ulcerative colitis. Surgery may remove a diseased segment or treat complications in Crohn's disease but does not eliminate the underlying tendency for inflammation elsewhere in the gastrointestinal tract; removing the colon and rectum can, however, be curative of the intestinal disease of ulcerative colitis.
Chronic inflammation can produce significant complications, although the risks vary by disease type and anatomical involvement. Crohn's disease can cause intestinal narrowing, obstruction, fistulas, abscesses, fissures, malnutrition and vitamin deficiencies. When Crohn's disease affects the colon, prolonged inflammation increases colorectal cancer risk, leading clinicians to consider surveillance colonoscopy after an appropriate duration of disease.
Ulcerative colitis can cause severe bleeding, profound anaemia, dehydration and, rarely, toxic megacolon, a serious complication in which severe inflammation causes marked colonic dilation and systemic illness. Both diseases can produce extraintestinal inflammatory problems and increase certain thrombotic risks. Disease activity and complication risk are therefore monitored even when symptoms seem controlled, because clinical symptoms do not always perfectly reflect the amount of intestinal inflammation. Maintaining remission is important both for quality of life and for reducing the cumulative consequences of uncontrolled inflammation.
IBD is consequently best understood as a chronic immune-mediated inflammatory disease of the gastrointestinal tract rather than a single condition with one mechanism or one treatment. Crohn's disease and ulcerative colitis share immune, genetic, microbial and environmental dimensions but have distinct distributions and patterns of tissue injury. IBD can be clinically mild, moderate or severe and can evolve over time, so management is based on objective disease assessment as well as symptoms.
It should not be confused with IBS, food intolerance or ordinary episodes of infectious gastroenteritis. At the same time, a person with IBD can also experience functional bowel symptoms resembling IBS even when inflammatory disease is controlled, which can make symptom interpretation difficult. Modern management therefore combines diagnosis of the exact IBD phenotype, control of inflammation, prevention and treatment of complications, nutritional and lifestyle support, appropriate cancer surveillance and long-term monitoring.
