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The concept of gluten allergy has been around for many years, but only recently has the term become ubiquitous. More patients than ever are entering clinics with self-diagnoses of various reactions to gluten, leaving practitioners to decipher the intricacies of gluten-induced symptoms. Adding to the confusion is the assumption by many that gluten allergy and its autoimmune counterpart, celiac disease, are the same disease entity. This article will attempt to clarify this misconception in the exploration of terminology, pathophysiology, changing clinical picture, and differential diagnosis using laboratory medicine.
Appropriate Terminology for Gluten-Related Disorders
Historically, one of the greatest impediments to accurate assessment and treatment of gluten-induced symptoms was the lack of a standardized diagnostic criteria for food allergy in general. In 2010, the National Institute of Allergy and Infectious Diseases sought to remedy this situation, creating a schematic for the wide scope of adverse food reactions. Two main subcategories created by the expert panel are immune-mediated and non-immune-mediated, the former encompassing food allergy and celiac disease and the latter encompassing food intolerances. Under these guidelines, any abnormal antibody response to gluten would be considered an allergy. However, the guidelines go on to assert that IgG antibodies should not be used to assess patients for food allergy. The use of IgA is implied in the schematic by the term non-IgE-mediated but is never discussed as a marker of food allergy.1
Immunoglobulins Produced in Response to Gluten Exposure
Some discourse still exists over whether non-IgE-mediated reactions should be considered true allergy. IgE has long been the standard laboratory measurement for classical allergic response, its secretion in the body resulting in histamine release by mast cells and potential anaphylaxis. Skin-prick testing is the most common form of assessing IgE-induced allergy symptoms, although serum IgE assays can also be used in conjunction with other clinical and laboratory measures to identify allergy in children and adults.1 IgE antibodies to gluten have been found in patients with atopic dermatitis and urticaria, and are beginning to be used in conjunction with skin-prick testing to diagnose classic wheat allergy.2
IgA is the first line of defense in mucosal immunity. It was discovered in the late 1960s that IgA-producing lymphocytes in the gastrointestinal system exist in 20 times greater quantity than those producing IgG. In a healthy gastrointestinal tract, enterocytes secrete IgA to inhibit colonization and invasion by various pathogens. IgA decreases antigen entry into the tissue space and activates lymphocytes; these cells then establish a common mucosal immunity by passage through the lymphatic system to other mucosal sites and subsequent secretion of antigen-specific IgA.3 Because of this common mucosal immunity, salivary IgA offers a convenient way to screen for immune-mediated reaction to gluten in patients hesitant to complete a comprehensive elimination-challenge diet. It has been shown that total IgA can be elevated during states of inflammation such as inflammatory bowel disease, metabolic syndrome, and connective tissue diseases.4,5 IgA has also been shown to be depressed in family history and/or clinical presentation of atopy, making it important to screen for total IgA when running allergen-specific IgA food allergy panels.6 Currently, anti-TTG IgA is used as the primary antibody in the diagnosis of celiac disease.7
In the greater medical community, IgG has yet to be established as a valid marker of food allergy. In fact, oral and sublingual allergy desensitization studies show that IgG antibody rises in conjunction with decreases in IgE mast cell reactivity and basophil responses.8 In line with this correlation, the European Academy of Allergy and Clinical Immunology (EAACI) asserts that IgG is more a marker of immunological tolerance than allergy.9 The National Institute of Allergy and Infectious Diseases lists allergen-specific IgG4 testing under the heading "Non-Standardized and Unproven Procedures" in its 2010 Guidelines for the Diagnosis and Management of Food Allergy in the United States.1
Despite the lack of wide-ranging acceptance for IgG-mediated food allergy, basic immunology tells us that in cases of mucosal endothelial cell destruction or when IgA is deficient, antigens from the lumen are complexed with IgG in the lamina propria.3 These immune complexes activate complement in a type III hypersensitivity reaction and result in temporary movement of inflammatory mediators and IgG into the lumen between epithelial cells. Although the exact role of IgG in gluten allergy has yet to be elucidated in research, the immunoglobulin's connection to enterocyte destruction and subsequent inflammation may explain why some patients' symptoms resolve when elimination of IgG-positive foods takes place. For example, one study showed that when patients with irritable bowel syndrome (IBS) removed IgG-positive foods from their diets, they experienced relief of symptoms.10 Furthermore, IgG to deamidated gluten peptide (DGP) and IgG to tissue transglutaminase (TTG) are used in cases of IgA deficiency under the new guidelines for diagnosis of celiac disease.7 Patients with gastrointestinal symptoms and IgG-positive gluten assays are undoubtedly mounting an immune response. Whether IgG is acting in immune tolerance to gluten or as an indicator of allergy, however, may currently be clearer in clinical practice than in research.
Celiac Disease: Pathophysiology, Changing Clinical Picture and Diagnostic Criteria
In celiac disease, gluten intake leads to both (1) production of antibodies against TTG and (2) inflammatory cytokine release leading to enterocyte destruction. The process begins with gluten entering the tissue space of the small intestine through either paracellular or transcellular absorption. Gluten is then deamidated, forming DGP, or cross-linked to TTG, forming gluten-TTG. In the presence of HLA-DQ2 or HLA-DQ8 cell surface markers, DGP and gluten-TTG are presented to CD4+ Th1 cells by dendritic cells, initiating a type IV hypersensitivity reaction. These CD4+ cells release IFN-gamma, which leads to the activation of the humoral immune response through the clonal expansion of B-cells. The resulting plasma cells produce IgA and IgG to gliadin and TTG. The tissue destruction component of this process is also perpetuated by IFN-gamma, which subsequently triggers lamina propria cells and fibroblasts to secrete matrix metalloproteinases. The metalloproteinases begin to degrade cellular matrix and basement membrane, while simultaneously enhancing the cytotoxicity of intraepithelial lymphocytes and NK cells. The latter facilitate apoptosis of enterocytes.11
Celiac disease is therefore a mix of humoral and cellular immune responses, mediated by antibodies and various cytokines. The condition develops due to multiple factors, including genetic susceptibility, presence of antibodies to TTG and/or DGP, intestinal damage, and gluten as an environmental immunological trigger. HLA-DQ2 is positive in 95% of those with biopsy-confirmed celiac disease, and the remaining 5% have HLA-DQ8.7 These genes must be present for autoimmunity to develop, as they are essential to the process of generation of anti-TTG/anti-DGP antibodies and enterocyte destruction. While absence of these markers can be helpful in exclusion of celiac disease from a list of differential diagnoses, the presence of either is not diagnostic as they are common in individuals of Caucasian European descent.3 Positive HLA-DQ2 is found in approximately 25% to 30% of these individuals, making the assay useful for – but not conclusive of – diagnosis of celiac disease. It is clear that celiac disease is a very specific, genetically influenced, autoimmune sequence of events within the umbrella of immune response to gluten, much as Hashimoto's thyroiditis exists within the overarching diagnostic category of thyroid disease.
Along with our understanding of genetic factors, the clinical picture of celiac disease is changing. Celiac disease was originally considered a childhood condition, but the mean age of diagnosis as of 2010 was 45 years.12 The condition may also be more common than most practitioners realize, as about 1 in 133 people in the US have the disease. In patients with a first-degree relative with celiac disease, prevalence increases to 1 in 22.13 Celiac disease was once considered an exclusively gastrointestinal disorder, but we now know the condition can manifest with extraintestinal symptoms such as ataxia, peripheral neuropathy, skin eruptions, anemia, muscle weakness, and osteopenia. 14,15 The disease also has associations with other autoimmune diagnoses, including but not limited to type 1 diabetes mellitus, idiopathic pulmonary hemosiderosis, systemic lupus erythmatosus, IgA nephropathy, polymyositis, and Sjögren's syndrome.12
New diagnostic criteria from the American College of Gastroenterology (ACG) recommend anti-TTG IgA as the most sensitive and specific serologic marker for celiac disease. They also assert the significance of assessing total IgA in the diagnostic process. Separate diagnostic guidelines are laid out for IgA deficiency and include assays of anti-TTG IgG and anti-DGP IgG. In children younger than 2 years of age, anti-TTG IgG alone or in conjunction with anti-DGP IgG should be used due to high probability of insufficient total IgA. HLA-DQ2 and HLA-DQ8 genetic haplotypes continue to be recommended. Antigliadin antibodies are no longer endorsed in establishing the diagnosis of celiac disease; however, confirmatory endoscopy and biopsy of the duodenum are still required. It is now necessary that 1 to 2 of the requisite biopsies be in the region of the duodenal bulb in order to identify an additional 9% to 13% of celiac disease patients.7 A positive intestinal biopsy will reveal villous atrophy.3
Differential Diagnosis in Gluten-Sensitive Individuals
Because the presenting symptoms of gluten-related conditions can be complex, laboratory medicine can be an important tool for differentiating between autoimmune, allergic, and functional conditions. The following are some of the more common diagnoses and laboratory measures to consider when encountering a patient with gluten-induced symptoms:
Inflammatory Bowel Disease
Clinical characteristics of inflammatory bowel disease (IBD) are often similar to those in celiac disease and such functional bowel disorders as irritable bowel syndrome (IBS). The diseases comprising IBD – Crohn's disease and ulcerative colitis – share the common symptoms of abdominal pain, diarrhea, fatigue, fever, weight loss, and possible blood in the stool. Endoscopy and colonoscopy with biopsy are the current standards of diagnosis for these conditions, but a fecal assay of calprotectin can serve as a relatively noninvasive way to distinguish patients urgently in need of biopsy from those with functional digestive issues.16 Calprotectin is a protein released from neutrophils during active inflammatory states, and has been correlated with a degree of intestinal inflammation. Patients between flares of the disease with elevated fecal calprotectin have been shown to be at greater risk of relapse within one year. Moreover, fecal calprotectin may indicate even subclinical mucosal inflammation, and therefore may help identify when an increase in naturopathic or conventional treatment is necessary. It should be noted that gastrointestinal bleeding has not been associated with levels of calprotectin, so clinical signs and symptoms must continue to be monitored to determine severity of disease progression.17
Eosinophilic Esophagitis
Eosinophilic esophagitis (EoE) is considered one disease within the spectrum of gluten-sensitive enteropathies. The clinical presentation of this condition can closely resemble that of celiac disease and includes abdominal pain, diarrhea, steatorrhea, and nausea and vomiting after meals. Weight loss is also common in adults and children. Eighty percent of patients with EoE will have symptoms of gastroesophageal reflux that do not respond to a 2-month trial of proton pump inhibitors (PPI).18 An endoscopy would be indicated in these cases; a diagnosis of EoE would be made if biopsy revealed greater than or equal to 15 eosinophils per high-power field.19
Wheat Allergy and Nonceliac Gluten Sensitivity
Some experts argue that celiac disease, wheat allergy, and gluten sensitivity are conditions characterized by three distinct immunological responses to gliadin protein with three separate histological and prognostic results. Wheat allergy is IgE-mediated and associated with allergic symptoms minutes to hours after exposure to gluten.2 Nonceliac gluten sensitivity (NCGS) is a diagnosis of exclusion to consider in patients with gluten-induced symptoms that improve on a gluten-free diet but lack genetic, immunologic, and endoscopic markers of celiac disease. Antigliadin IgA or IgG may be present in this condition.20 NCGS is not typically associated with intestinal damage and permeability, in contrast to the overt enterocyte destruction that occurs in celiac disease. The elevated fecal lactoferrin level and lactulose/mannitol ratio frequently seen in IBD and celiac disease are typically normal in NCGS.21
Irritable Bowel Syndrome
Irritable bowel syndrome can manifest as reactivity to multiple foods, including NCGS.21 The diagnosis of IBS is currently considered one of exclusion, but does have its own specific Rome III diagnostic criteria. According to these guidelines, a patient must have recurrent abdominal pain or discomfort (an uncomfortable sensation not described as pain) for at least 3 days per month in the last 3 months. This abdominal pain or discomfort must be associated with two or more of the following characteristics: improvement with defecation, onset associated with a change in stool frequency, or onset associated with a change in form (appearance) of stool. Moreover, the criteria must have been fulfilled for the last 3 months with symptom onset at least 6 months prior to diagnosis.22 Laboratory measures to rule out autoimmune and inflammatory conditions may include fecal calprotectin, an iron panel to assess for anemia, food immunoglobulin testing, stool testing for parasites and intestinal bacterial overgrowth, celiac disease markers, and intestinal biopsy.
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