What is the difference between a food allergy and a food intolerance?

What is the difference between a food allergy and a food intolerance?
They both involve food, but other than that, food allergies and food intolerances have little in common.
Food allergies are orchestrated by the body’s immune system. Food intolerance results from the gut’s inability to digest food normally. Food allergies can be fatal; food intolerance causes discomfort but is not usually serious.
Food allergies require eliminating all traces of the food from your diet. Food intolerances can be managed without such drastic measures. Here are illustrations of an allergic reaction and a food intolerance:
Lactose intolerance: A lack of lactase
Lactose intolerance is one of the most common types of food intolerance. Normally, an enzyme in the small intestine called lactase (1) breaks down lactose into glucose and galactose. People who are lactose intolerant don’t make sufficient amounts of lactase, so lactose passes intact through the small intestine into the large intestine and draws water into the intestine. There, bacteria break down (ferment) lactose (2), generating hydrogen and carbon dioxide gases, as well as lactic acid and acetic acid, which irritate the stomach lining. The resulting symptoms — gas, abdominal pain, cramping, and diarrhea — usually occur 30 minutes to two hours after you drink or eat foods containing lactose.
1. First exposure to allergen
Dendritic cells begin the innate immune response, recognizing an allergen as an invader. They process the invader and display a recognizable portion as an antigen, which activates Th2 cells. This sets off a complex chain reaction involving the release of cytokines, chemicals that signal B cells to produce IgE antibodies that will be ready for the allergen the next time it makes its appearance.
2. Next exposure to allergen
The IgE antibodies created on first exposure to the allergen lie in wait on the surface of mast cells, immune system cells found in the mucous membrane layers at the entry points of the body (such as the nose, eyes, lungs, and gut). When an allergen meets up with the IgE antibodies, the mast cell releases immune system chemicals such as tryptase, histamine, leukotrienes, and prostaglandins.
Mast cells also produce their own cytokines that stimulate B cells to produce more IgE, which intensifies the allergic response. At the same time, other cytokines recruit other immune cells, known as eosinophils, to the site of the allergic response, setting up local inflammation. Meanwhile, the allergen continues to stimulate the ever-vigilant Th2 cells, stirring up more production of IgE and inflammation and fueling the allergic reaction even further.
A food allergy is the immune system’s overreaction to a normally harmless food. The most common foods that people are allergic to are milk, eggs, peanuts, tree nuts, fish, shellfish, wheat and soy.
When someone with a food allergy eats an offending food, the immune system springs into action. An antibody called IgE signals immune cells to release chemicals that stimulate nerves, dilate blood vessels and cause inflammation. This can cause lightheadedness; itching, hives or rash; swelling of the lips, tongue and throat; and nausea, vomiting, cramping and diarrhea.
Rarely, a food allergy will trigger a life-threatening, whole-body reaction known as anaphylaxis. That’s one of the reasons that some airlines no longer hand out peanuts on flights. People with food allergies must completely avoid the dietary culprit.
Food intolerance results from the body’s inability to properly digest or metabolize a food. Symptoms include gas, bloating, nausea and diarrhea. Lactose, a milk sugar, and gluten, a protein in grains, are the substances that people are most likely to be intolerant or sensitive to. Intolerance to lactose leads to cramping pain in the abdomen and loose bowel movements.
The reaction to gluten can range from mild to severe. With severe intolerance, the intestines produce so much uncontrollable diarrhea that a person can suffer from severe dehydration — severe enough that if the fluid is not promptly replaced, they can die.
One common and effective way to reduce symptoms is enzyme supplementation. Say you’re lactose intolerant. This means you don’t produce enough lactase, the enzyme that breaks down lactose, or milk sugar, into smaller, more easily digestible molecules. Taking a supplement that contains lactase can help you digest the lactose in dairy foods.
Probiotics might also help. There’s some evidence that ingesting “gut-friendly” bacteria may help relieve lactose intolerance. However, there’s no standard formulation for probiotics, and finding one that’s right for you can be a hit-or-miss affair.
There is no similar antidote to gluten. A person with gluten intolerance needs to avoid any foods that contain gluten. Better labeling of gluten on food packages and the creation of more gluten-free products in recent years has made life somewhat better for gluten-intolerant people. more  

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