Crohn's

Crohn's disease is a type of inflammatory bowel disease (IBD) that may affect any segment of the gastrointestinal tract. Symptoms often include abdominal pain, diarrhea (which may be bloody if inflammation is severe), fever, abdominal distension, and weight loss. Complications outside of the gastrointestinal tract may include anemia, skin rashes, arthritis, inflammation of the eye, and fatigue. The skin rashes may be due to infections as well as pyoderma gangrenosum or erythema nodosum. Bowel obstruction may occur as a complication of chronic inflammation, and those with the disease are at greater risk of colon cancer and small bowel cancer.

While the precise causes of Crohn's disease (CD) are unknown, it is believed to be caused by a combination of environmental, immune, and bacterial factors in genetically susceptible individuals. It results in a chronic inflammatory disorder, in which the body's immune system defends the gastrointestinal tract, possibly targeting microbial antigens. While Crohn's is an immune-related disease, it does not appear to be an autoimmune disease (in that the immune system is not being triggered by the body itself). The exact underlying immune problem is not clear; however, it may be an immunodeficiency state.

About half of the overall risk is related to genetics, with more than 70 genes having been found to be involved. Tobacco smokers are twice as likely to develop Crohn's disease as nonsmokers. It also often begins after gastroenteritis. Diagnosis is based on a number of findings, including biopsy and appearance of the bowel wall, medical imaging, and description of the disease. Other conditions that can present similarly include irritable bowel syndrome and Behçet's disease.

There is no known cure for Crohn's disease. Treatment options are intended to help with symptoms, maintain remission, and prevent relapse. In those newly diagnosed, a corticosteroid may be used for a brief period of time to rapidly improve symptoms, alongside another medication such as either methotrexate or a thiopurine used to prevent recurrence. Stopping smoking is recommended in people with Crohn's disease. One in five people with the disease is admitted to the hospital each year, and half of those with the disease will require surgery for the disease at some point over a ten-year period. While surgery should be used as little as possible, it is necessary to address some abscesses, certain bowel obstructions, and cancers. Checking for bowel cancer via colonoscopy is recommended every few years, starting eight years after the disease has begun.

Crohn's disease affects about 3.2 per 1,000 people in Europe, North America, and the UK. It is less common in Asia and Africa. It has historically been more common in the developed world. Rates have, however, been increasing, particularly in the developing world, since the 1970s. Inflammatory bowel disease resulted in 47,400 deaths in 2015, and those with Crohn's disease have a slightly reduced life expectancy. It tends to start in young adulthood, though it can occur at any age. Males and females are equally affected.

Name

The disease was named after gastroenterologist Burrill Bernard Crohn, who in 1932, together with two colleagues at Mount Sinai Hospital in New York, described a series of patients with inflammation of the terminal ileum of the small intestine, the area most commonly affected by the illness.

Signs and symptoms

Gastrointestinal

Many people with Crohn's disease have symptoms for years before the diagnosis. The usual onset is in the teens and twenties, but can occur at any age. Because of the 'patchy' nature of the gastrointestinal disease and the depth of tissue involvement, initial symptoms can be more subtle than those of ulcerative colitis. People with Crohn's disease experience chronic recurring periods of flare-ups and remission. The symptoms experienced can change over time as inflammation increases and spreads. Symptoms can also be different depending on which organs are involved. It is generally thought that the presentation of Crohn's disease is different for each patient due to the high variability of symptoms, organ involvement, and initial presentation.

Perianal

Perianal discomfort may also be prominent in Crohn's disease. Itchiness or pain around the anus may be suggestive of inflammation of the anus, or perianal complications such as anal fissures, fistulae, or abscesses around the anal area. Perianal skin tags are also common in Crohn's disease, and may appear with or without the presence of colorectal polyps. Fecal incontinence may accompany perianal Crohn's disease.

Intestines

The intestines, especially the colon and terminal ileum, are the most commonly affected areas of the body. Abdominal pain is a common initial symptom of Crohn's disease, especially in the lower right abdomen. Flatulence, bloating, and abdominal distension are additional symptoms and may also add to the intestinal discomfort. Pain is often accompanied by diarrhea, which may or may not be bloody. Inflammation in different areas of the intestinal tract can affect the quality of the feces. Ileitis typically results in large-volume, watery feces, while colitis may result in a smaller volume of feces of higher frequency. Fecal consistency may range from solid to watery. In severe cases, an individual may have more than 20 bowel movements per day, and may need to awaken at night to defecate. Visible bleeding in the feces is less common in Crohn's disease than in ulcerative colitis, but is not unusual. Bloody bowel movements are usually intermittent, and may be bright red, dark maroon, or even black in color. The color of bloody stool depends on the location of the bleed. In severe Crohn's colitis, bleeding may be copious.

Stomach and esophagus

The stomach is rarely the sole or predominant site of CD. To date there are only a few documented case reports of adults with isolated gastric CD and no reports in the pediatric population. Isolated stomach involvement is very unusual presentation accounting for less than 0.07% of all gastrointestinal CD. Rarely, the esophagus and stomach may be involved in Crohn's disease. These can cause symptoms including difficulty swallowing (dysphagia), upper abdominal pain, and vomiting.

Oropharynx (mouth)

The mouth may be affected by recurrent sores ( aphthous ulcers). Recurrent aphthous ulcers are common; however, it is not clear whether this is due to Crohn's disease or simply that they are common in the general population. Other findings may include diffuse or nodular swelling of the mouth, a cobblestone appearance inside the mouth, granulomatous ulcers, or pyostomatitis vegetans. Medications that are commonly prescribed to treat CD, such as anti-inflammatory and sulfa-containing drugs, may cause lichenoid drug reactions in the mouth. Fungal infection such as candidiasis is also common due to the immunosuppression required in the treatment of the disease. Signs of anemia such as pallor and angular cheilitis or glossitis are also common due to nutritional malabsorption.

People with Crohn's disease are also susceptible to angular stomatitis, an inflammation of the corners of the mouth, and pyostomatitis vegetans.

Systemic

Like many other chronic, inflammatory diseases, Crohn's disease can cause a variety of systemic symptoms. Among children, growth failure is common. Many children are first diagnosed with Crohn's disease based on inability to maintain growth. As it may manifest at the time of the growth spurt in puberty, up to 30% of children with Crohn's disease may have retardation of growth. Fever may also be present, though fevers greater than 38.5 °C (101.3 °F) are uncommon unless there is a complication such as an abscess. Among older individuals, Crohn's disease may manifest as weight loss, usually related to decreased food intake, since individuals with intestinal symptoms from Crohn's disease often feel better when they do not eat and might lose their appetite. People with extensive small intestine disease may also have malabsorption of carbohydrates or lipids, which can further exacerbate weight loss.

Extraintestinal

Crohn's disease can affect many organ systems beyond the gastrointestinal tract.

Visual

Inflammation of the interior portion of the eye, known as uveitis, can cause blurred vision and eye pain, especially when exposed to light ( photophobia). Uveitis can lead to loss of vision if untreated.

Inflammation may also involve the white part of the eye (sclera) or the overlying connective tissue ( episclera), which causes conditions called scleritis and episcleritis, respectively.

Other very rare ophthalmological manifestations include: conjunctivitis, glaucoma, and retinal vascular disease.

Gallbladder and liver

Crohn's disease that affects the ileum may result in an increased risk of gallstones. This is due to a decrease in bile acid resorption in the ileum, and the bile gets excreted in the stool. As a result, the cholesterol/bile ratio increases in the gallbladder, resulting in an increased risk for gallstones. Although the association is greater in the context of ulcerative colitis, Crohn's disease may also be associated with primary sclerosing cholangitis, a type of inflammation of the bile ducts.

Liver involvement of Crohn's disease can include cirrhosis and steatosis. Nonalcoholic fatty liver disease (Nonalcoholic Steatohepatitis, NAFLD) are relatively common and can slowly progress to end-stage liver disease. NAFLD sensitizes the liver to injury and increases the risk of developing acute or chronic liver failure following another liver injury.

Other rare hepatobiliary manifestations of Crohn's disease include: cholangiocarcinoma, granulomatous hepatitis, cholelithiasis, autoimmune hepatitis, hepatic abscess, and pericholangitis.

Renal and urological

Nephrolithiasis, obstructive uropathy, and fistulization of the urinary tract directly result from the underlying disease process. Nephrolithiasis is due to calcium oxalate or uric acid stones. Calcium oxalate is due to hyperoxaluria typically associated with either distal ileal CD or ileal resection. Oxalate absorption increases in the presence of unabsorbed fatty acids in the colon. The fatty acids compete with oxalate to bind calcium, displacing the oxalate, which can then be absorbed as unbound sodium oxalate across colonocytes and excreted into the urine. Because sodium oxalate only is absorbed in the colon, calcium-oxalate stones form only in patients with an intact colon. Patients with an ileostomy are prone to formation of uric-acid stones because of frequent dehydration. The sudden onset of severe abdominal, back, or flank pain in patients with IBD, particularly if different from the usual discomfort, should lead to inclusion of a renal stone in the differential diagnosis.

Urological manifestations in patients with IBD may include ureteral calculi, enterovesical fistula, perivesical infection, perinephric abscess, and obstructive uropathy with hydronephrosis. Ureteral compression is associated with retroperitoneal extension of the phlegmonous inflammatory process involving the terminal ileum and cecum, and may result in hydronephrosis severe enough to cause hypertension.

Immune complex glomerulonephritis presenting with proteinuria and hematuria has been described in children and adults with CD or UC. Diagnosis is by renal biopsy, and treatment parallels the underlying IBD.

Amyloidosis (see endocrinological involvement) secondary to Crohn's disease has been described and is known to affect the kidneys.

Pancreatic

Pancreatitis may be associated with both UC and CD. The most common cause is iatrogenic and involves sensitivity to medications used to treat IBD (3% of patients), including sulfasalazine, mesalamine, 6-mercaptopurine, and azathioprine. Pancreatitis may present as symptomatic (in 2%) or more commonly asymptomatic (8–21%) disease in adults with IBD.

Cardiovascular and circulatory

Children and adults with IBD have been rarely (<1%) reported developing pleuropericarditis either at initial presentation or during active or quiescent disease. The pathogenesis of pleuropericarditis is unknown, although certain medications (e.g., sulfasalazine and mesalamine derivatives) have been implicated in some cases. The clinical presentation may include chest pain, dyspnea, or in severe cases pericardial tamponade requiring rapid drainage. Nonsteroidal anti-inflammatory drugs have been used as therapy, although this should be weighed against the hypothetical risk of exacerbating the underlying IBD.

In rare cases, cardiomyopathy, endocarditis, and myocarditis have been described.

Crohn's disease also increases the risk of blood clots; painful swelling of the lower legs can be a sign of deep venous thrombosis, while difficulty breathing may be a result of pulmonary embolism.

Respiratory

Laryngeal involvement in inflammatory bowel disease is extremely rare. Only 12 cases of laryngeal involvement in Crohn disease have been reported until now. Moreover, only one case of laryngeal manifestations in ulcerative colitis has been reported so far. 9 patients complained of difficulty in breathing due to edema and ulceration from the larynx to the hypopharynx Hoarseness, sore throat, and odynophagia are other symptoms of laryngeal involvement of Crohn's disease.

Considering extraintestinal manifestations of CD, those involving the lung are relatively rare. However, there is a wide array of lung manifestations, ranging from subclinical alterations, airway diseases and lung parenchymal diseases to pleural diseases and drug-related diseases. The most frequent manifestation is bronchial inflammation and suppuration with or without bronchiectasis. There are a number of mechanisms by which the lungs may become involved in CD. These include the same embryological origin of the lung and gastrointestinal tract by ancestral intestine, similar immune systems in the pulmonary and intestinal mucosa, the presence of circulating immune complexes and auto-antibodies, and the adverse pulmonary effects of some drugs. A complete list of known pulmonary manifestations include: Fibrosing alveolitis, Pulmonary vasculitis, Apical fibrosis, Bronchiectasis, Bronchitis, Bronchiolitis, Tracheal stenosis, Granulomatous lung disease, and Abnormal pulmonary function.

Musculoskeletal

Crohn's disease is associated with a type of rheumatologic disease known as seronegative spondyloarthropathy. This group of diseases is characterized by inflammation of one or more joints (arthritis) or muscle insertions (enthesitis). The arthritis in Crohn's disease can be divided into two types. The first type affects larger weight-bearing joints such as the knee (most common), hips, shoulders, wrists, or elbows. The second type symmetrically involves five or more of the small joints of the hands and feet. The arthritis may also involve the spine, leading to ankylosing spondylitis if the entire spine is involved, or simply sacroiliitis if only the sacroiliac joint is involved. The symptoms of arthritis include painful, warm, swollen, stiff joints, and loss of joint mobility or function.

Crohn's disease increases the risk of osteoporosis or thinning of the bones. Individuals with osteoporosis are at increased risk of bone fractures.

Dermatological

Crohn's disease may also involve the skin, blood, and endocrine system. Erythema nodosum is the most common type of skin problem, occurring in around 8% of people with Crohn's disease, producing raised, tender red nodules usually appearing on the shins. Erythema nodosum is due to inflammation of the underlying subcutaneous tissue, and is characterized by septal panniculitis.

Pyoderma gangrenosum is a less common skin problem, occurring in under 2%, and is typically a painful ulcerating nodule.

Clubbing, a deformity of the ends of the fingers, may also be a result of Crohn's disease.

Other very rare dermatological manifestations include: Pyostomatitis vegetans, Psoriasis, Erythema multiforme, Epidermolysis bullosa acquista (described in a case report), and Metastatic CD (the spread of Crohn's inflammation to the skin). It is unknown if Sweet's Syndrome is connected to Crohn's disease.

Neurological

Crohn's disease can also cause neurological complications (reportedly in up to 15%).Crohn's disease
. professionals.epilepsy.com. Retrieved July 13, 2007. The most common of these are seizures, stroke, myopathy, peripheral neuropathy, headache, and depression.

Central and peripheral neurological disorders are described in patients with IBD and include peripheral neuropathies, myopathies, focal central nervous system defects, convulsions, confusional episodes, meningitis, syncope, optic neuritis, and sensorineural loss. Autoimmune mechanisms are proposed for involvement with IBD. Nutritional deficiencies associated with neurological manifestations, such as vitamin B12 deficiency, should be investigated. Spinal abscess has been reported in both a child and an adult with initial complaints of severe back pain due to extension of a psoas abscess from the epidural space to the subarachnoid space.

Psychiatric and psychological

Crohn's disease is linked to many psychological disorders, including depression and anxiety, denial of your disease, the need for dependence or dependent behaviors, feeling overwhelmed, and having a poor self-image.

Endocrinological or hematological

Autoimmune hemolytic anemia, a condition in which the immune system attacks the red blood cells, is also more common in Crohn's disease and may cause fatigue, a pale appearance, and other symptoms common in anemia.

Secondary amyloidosis (AA) is another rare but serious complication of inflammatory bowel disease (IBD), generally seen in Crohn's disease. At least 1% of patients with Crohn's disease develop amyloidosis. In the literature, the time lapse between the onset of Crohn's disease and the diagnosis of amyloidosis has been reported to range from one to 21 years.

Leukocytosis and thrombocytopenia are usually due to immunosuppressant treatments or sulfasalazine. Plasma erythropoietin levels often are lower in patients with IBD than expected, in conjunction with severe anemia.

Thrombocytosis and thromboembolic events resulting from a hypercoagulable state in patients with IBD can lead to pulmonary embolism or thrombosis elsewhere in the body. Thrombosis has been reported in 1.8% of patients with UC and 3.1% of patients with CD. Thromboembolism and thrombosis are less frequently reported among pediatric patients, with 3 patients with UC and 1 with CD described in case reports.

In rare cases, hypercoagulation disorders and portal vein thrombosis have been described.

Malnutrition symptoms

People with Crohn's disease may develop anemia due to vitamin B₁₂, folate, iron deficiency, or due to anemia of chronic disease. The most common is iron deficiency anemia from chronic blood loss, reduced dietary intake, and persistent inflammation leading to increased hepcidin levels, restricting iron absorption in the duodenum. As Crohn's disease most commonly affects the terminal ileum where the vitamin B12/intrinsic factor complex is absorbed, B12 deficiency may be seen. This is particularly common after surgery to remove the ileum. Involvement of the duodenum and jejunum can impair the absorption of many other nutrients including folate. People with Crohn's often also have issues with small bowel bacterial overgrowth syndrome, which can produce micronutrient deficiencies. Text was copied from this source, which is available under
Creative Commons Attribution 2.0 Generic (CC BY 2.0)
license.

Complications

Intestinal damage

Crohn's disease can lead to several mechanical complications within the intestines, including obstruction, fistulae, and abscesses. Obstruction typically occurs from strictures or adhesions that narrow the lumen, blocking the passage of the intestinal contents. A fistula can develop between two loops of bowel, between the bowel and bladder, between the bowel and vagina, and between the bowel and skin. Abscesses are walled-off concentrations of infection, which can occur in the abdomen or in the perianal area. Crohn's is responsible for 10% of vesicoenteric fistulae, and is the most common cause of ileovesical fistulae.

Symptoms caused by intestinal stenosis, or the tightening and narrowing of the bowel, are also common in Crohn's disease. Abdominal pain is often most severe in areas of the bowel with stenosis. Persistent vomiting and nausea may indicate stenosis from small bowel obstruction or disease involving the stomach, pylorus, or duodenum.

Intestinal granulomas are a walled-off portions of the intestine by macrophages in order to isolate infections. Granuloma formation is more often seen in younger patients, and mainly in the severe, active penetrating disease. Granuloma is considered the hallmark of microscopic diagnosis in Crohn's disease (CD), but granulomas can be detected in only 21-60% of CD patients.

Cancer

Crohn's disease also increases the risk of cancer in the area of inflammation. For example, individuals with Crohn's disease involving the small bowel are at higher risk for small intestinal cancer. Similarly, people with Crohn's colitis have a relative risk of 5.6 for developing colon cancer. Screening for colon cancer with colonoscopy is recommended for anyone who has had Crohn's colitis for at least eight years.

Some studies suggest there is a role for chemoprotection in the prevention of colorectal cancer in Crohn's involving the colon; two agents have been suggested, folate and mesalamine preparations. Also, immunomodulators and biologic agents used to treat this disease may promote developing extra-intestinal cancers.

Some cancers, such as Acute Myelocytic Leukaemia have been described in cases of Crohn's disease. Hepatosplenic T-cell lymphoma (HSTCL) is a rare, lethal disease generally seen in young male patients with inflammatory bowel disease. TNF-α Inhibitor treatments ( infliximab, adalimumab, certolizumab, natalizumab, and etanercept) are thought to be the cause of this rare disease.

Major complications

Major complications of Crohn's disease include bowel obstruction, abscesses, free perforation, and hemorrhage, which in rare cases may be fatal.

Other complications

Individuals with Crohn's disease are at risk of malnutrition for many reasons, including decreased food intake and malabsorption. The risk increases following resection of the small bowel. Such individuals may require oral supplements to increase their caloric intake, or in severe cases, total parenteral nutrition (TPN). Most people with moderate or severe Crohn's disease are referred to a dietitian for assistance in nutrition.

Small intestinal bacterial overgrowth (SIBO) is characterized by excessive proliferation of colonic bacterial species in the small bowel. Potential causes of SIBO include fistulae, strictures, or motility disturbances. Hence, patients with Crohn's disease are especially predisposed to develop SIBO. As result, CD patients may experience malabsorption and report symptoms such as weight loss, watery diarrhea, meteorism, flatulence, and abdominal pain, mimicking acute flare in these patients.

Pregnancy

Crohn's disease can be problematic during pregnancy, and some medications can cause adverse outcomes for the fetus or mother. Consultation with an obstetrician and gastroenterologist about Crohn's disease and all medications facilitates preventive measures. In some cases, remission occurs during pregnancy. Certain medications can also lower sperm count or otherwise adversely affect a man's fertility.

Ostomy-related complications

Common complications of an ostomy (a common surgery in Crohn's disease) are: Mucosal edema, Peristomal dermatitis, Retraction, Ostomy prolapse, Mucosal/skin detachment, Hematoma, Necrosis, Parastomal hernia, and Stenosis.

Etiology

The etiology of Crohn's disease is unknown. Many theories have been disputed, with four main theories hypothesized to be the primary mechanism of Crohn's disease. In autoimmune diseases, antibodies and T lymphocytes are the primary mode of inflammation. These cells and bodies are part of the adaptive immune system, or the part of the immune system that learns to fight foreign bodies when first identified. Autoinflammatory diseases are diseases where the innate immune system, or the immune system we are genetically coded with, is designed to attack our own cells. Crohn's disease likely has involvement of both the adaptive and innate immune systems.

Autoinflammatory theory

Crohn's disease can be described as a multifactorial autoinflammatory disease. The etiopathogenesis of Crohn's disease is still unknown. In any event, a loss of the regulatory capacity of the immune apparatus would be implicated in the onset of the disease. In this respect interestingly enough, as for Blau's disease (a monogenic autoinflammatory disease), the NOD2 gene mutations have been linked to Crohn's disease. However, in Crohn's disease, NOD2 mutations act as a risk factor, being more common among Crohn's disease patients than the background population, while in Blau's disease NOD2 mutations are linked directly to this syndrome, as it is an autosomal-dominant disease. All this new knowledge in the pathogenesis of Crohn's disease allows us to put this multifactorial disease in the group of autoinflammatory syndromes.

Some examples of how the innate immune system affects bowel inflammation have been described. A meta-analysis of CD genome-wide association studies revealed 71 distinct CD-susceptibility loci. Interestingly, three very important CD-susceptibility genes (the intracellular pathogen-recognition receptor, NOD2; the autophagy-related 16-like 1, ATG16L1 and the immunity-related GTPase M, IRGM) are involved in innate immune responses against gut microbiota, while one (the X-box binding protein 1) is involved in regulation of the daptiveimmune pathway via MHC class II, resulting in autoinflammatory inflammation. Studies have also found that increased ILC3 can overexpress major histocompatibility complex (MHC) II. MHC class II can induce CD4+ T cell apoptosis, thus avoiding the T cell response to normal bowel micro bacteria. Further studies of IBD patients compared with non-IBD patients found that the expression of MHC II by ILC3 was significantly reduced in IBD patients, thus causing an immune reaction against intestinal cells or normal bowel bacteria and damaging the intestines. This can also make the intestines more susceptible to environmental factors, such as food or bacteria.

The thinking is, that because Crohn's disease has strong innate immune system involvement and has NOD2 mutations as a predisposition, Crohn's disease is more likely an autoinflammatory disease than an autoimmune disease.

Immunodeficiency theory

A substantial body of data has emerged in recent years to suggest that the primary defect in Crohn's disease is actually one of relative immunodeficiency. This view has been bolstered recently by novel immunological and clinical studies that have confirmed gross aberrations in this early response, consistent with subsequent genetic studies that highlighted molecules important for innate immune function. The suggestion therefore is that Crohn's pathogenesis actually results from partial immunodeficiency, a theory that coincides with the frequent recognition of a virtually identical, non-infectious inflammatory bowel disease arising in patients with congenital monogenic disorders impairing phagocyte function.

Causes

While the exact cause or causes are unknown, Crohn's disease seems to be due to a combination of environmental factors and genetic predisposition. Crohn's is the first genetically complex disease in which the relationship between genetic risk factors and the immune system is understood in considerable detail. Each individual risk mutation makes a small contribution to the overall risk of Crohn's (approximately 1:200). The genetic data, and direct assessment of immunity, indicates a malfunction in the innate immune system.
In this view, the chronic inflammation of Crohn's is caused when the adaptive immune system tries to compensate for a deficient innate immune system.

Genetics

Crohn's has a genetic component. Because of this, siblings of known people with Crohn's are 30 times more likely to develop Crohn's than the general population.

The first mutation found to be associated with Crohn's was a frameshift in the NOD2 gene (also known as the CARD15 gene), followed by the discovery of point mutations. Over 30 genes have been associated with Crohn's; a biological function is known for most of them. For example, one association is with mutations in the XBP1 gene, which is involved in the unfolded protein response pathway of the endoplasmic reticulum. The gene variants of NOD2/CARD15 seem to be related with small-bowel involvement. Other well documented genes which increase the risk of developing Crohn's disease are ATG16L1, IL23R, IRGM, and SLC11A1.
There is considerable overlap between susceptibility loci for IBD and mycobacterial infections. Genome-wide association studies have shown that Crohn's disease is genetically linked to coeliac disease.

Crohn's has been linked to the gene LRRK2 with one variant potentially increasing the risk of developing the disease by 70%, while another lowers it by 25%. The gene is responsible for making a protein, which collects and eliminates waste product in cells, and is also associated with Parkinson's disease.

Immune system

There was a prevailing view that Crohn's disease is a primary T cell autoimmune disorder; however, a newer theory hypothesizes that Crohn's results from an impaired innate immunity. The later hypothesis describes impaired cytokine secretion by macrophages, which contributes to impaired innate immunity and leads to a sustained microbial-induced inflammatory response in the colon, where the bacterial load is high. Another theory is that the inflammation of Crohn's was caused by an overactive T_h1 and T_h17 cytokine response.

In 2007, the ATG16L1 gene was implicated in Crohn's disease, which may induce autophagy and hinder the body's ability to attack invasive bacteria. Another study theorized that the human immune system traditionally evolved with the presence of parasites inside the body and that the lack thereof due to modern hygiene standards has weakened the immune system. Test subjects were reintroduced to harmless parasites, with positive responses.

Microbes

It is hypothesized that maintenance of commensal microorganism growth in the GI tract is dysregulated, either as a result or cause of immune dysregulation.

There is an apparent connection between Crohn's disease, ''Mycobacterium'', other pathogenic bacteria, and genetic markers. A number of studies have suggested a causal role for ''Mycobacterium avium'' subspecies ''paratuberculosis'' (MAP), which causes a similar disease, Johne's disease, in cattle. In many individuals, genetic factors predispose individuals to ''Mycobacterium avium'' subsp.'' paratuberculosis'' infection. This bacterium may produce certain compounds containing mannose, which may protect both itself and various other bacteria from phagocytosis, thereby possibly causing a variety of secondary infections.

NOD2 is a gene involved in Crohn's genetic susceptibility. It is associated with macrophages' diminished ability to phagocytize MAP. This same gene may reduce innate and adaptive immunity in gastrointestinal tissue and impair the ability to resist infection by the MAP bacterium. Macrophages that ingest the MAP bacterium are associated with high production of TNF-α.

Other studies have linked specific strains of entero