They have a handful functions, still one of the primary ones is to patrol the blood and lymphatic fluid looking for foreign substances. Every cell in our bodies has a genetic 'signature' that is unique to us. Anything, like a bacteria or virus, that has a different signature is seen because a foreign substance and a candidate for removal. ... read more
All allergic symptoms are mostly an immune format overreaction to an allergen, typically in the environment. On the other hand, there are genetic factors that predispose a person to allergies. Merely having one or more of the symptoms described above doesn't necessarily indicate the presence of an allergy. Though, when they persist for more than two weeks, it's appropriate to strive a professional diagnosis. ... read more
"To do that, white blood cells move to an area where the foreign substance is unearthed. In the case of an allergen, those are usually proteins such seeing as pollen or those uncovered in certain foods like milk or peanuts. The white blood cells then cause the release of histamines and enzymes into the surrounding fluid to envelop the foreign protein."

Tonsils, the appendix and others are too lymphoid tissue. Conversely there is so-called lymphoid tissue elsewhere, including nearly all importantly the bone marrow and the thymus (an organ just behind the breastbone). It is the thymus that produces T cells, a type of white blood cell. These T cells play a key role in allergies.
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