When the immune system overreacts due to certain substances that typically do not cause any reaction in most people, it is known as allergy. It is a type of hypersensitivity, and allergens is the name given to the substances that trigger these overreactions. As per the data released by the American Academy of Allergy, Asthma, and Immunology (AAAAI), globally, there has been a steady rise in the percentage of people suffering from some kind of allergy over the last 50 years. It is estimated that almost 50% of children in the whole world exhibit overreaction to certain types of vegetation, animals, birds, insects, or foods.
Allergies can happen to anyone, but people who already have family members suffering from some kind of allergy are at a higher risk. Also, an individual predisposed to allergy is not necessarily going to react to the same allergen that causes allergy in their parents or siblings. The reaction depends on the allergens a person is exposed to, as well as the response of the immune system of that individual.
Hypersensitivities are categorized into four types, I through IV. The categories are defined, to an extent, as per the time it takes for the allergic reaction to occur as well as the parts of the immune system that are activated.
Allergies are commonly associated with these two types of hypersensitivities:
People with this type of hypersensitivity produce antibodies (an immune protein) when they are exposed to an allergen. This antibody produced in reaction to the exposure is classified as immunoglobulin E. These antibodies attach to the allergens, and that leads to local reactions. There are times when these produce a reaction in the whole body, and it typically happens within minutes.
These are delayed allergic reactions that happen due to the interaction of the allergens with very specific immune cells that are also known as white blood cells or T lymphocytes.
Type I Allergy
This type typically affects the gastrointestinal and respiratory systems along with the skin of the individual. When a predisposed person is exposed to a potential allergen for the first time, their body does not produce an immediate reaction. However, the body begins to produce the immunoglobulin E antibodies that are designed to work against that specific allergen. It is also known as allergen specific IgE antibody. It sensitizes that individual and any subsequent exposure to that allergen might cause severe reactions.
The IgE antibody produced by the body when exposed to an allergen binds itself to mast cells (specialized cells residing in the tissues) along with basophils in the bloodstream. This leads to the priming of the immune system. When the body is exposed subsequently to the same allergens, this antibody recognizes that allergen and binds to it. This binding triggers the release of certain chemicals, including histamine, which is responsible for allergic symptoms. These reactions occur in the nose, mouth, or on the skin, depending on the place where the allergen has interacted with the body.
The severity of type I allergic reactions can vary a lot. It might lead to only hives or red rashes at one time but can also lead to a life-threatening reaction known as anaphylaxis, the next time. Anaphylaxis is the name given to a multi-organ reaction. It typically starts with agitation, and other symptoms include pale skin caused by low blood pressure, a feeling of impending doom combined with a loss of consciousness sometimes. Without quick administration of epinephrine injection (adrenaline), this condition can turn fatal.
A variety of substances can trigger type I allergies. Here is a list of a few common substances:
- Various plants, including grasses, weeds, and pollens, various foods, dust mites, insect venoms, mold spores, saliva and dander from animals such as cats and dogs, various occupational substances such as latex, and even drugs such as penicillin.
- Cross-reactions can also happen, which means an individual who is allergic to ragweed might also be allergic to another substance such as bananas or melons such as cantaloupe or watermelon. Some of the most common causes of severe allergic reactions to food items include shellfish, tree nuts including walnuts as well as peanuts.
Type IV Allergy
This type of allergy typically involve the skin. These are delayed hypersensitivities as the allergic reaction takes around 48 to 72 hours to surface after exposure to a certain allergen. These allergic reactions are caused when they allergen encounters specific sensitized T lymphocytes cells. These lymphocyte cells then release various toxic and inflammatory substances that cause other white blood cells in the body to rush to the site of exposure, and this results in tissue injury. This type of allergy does not require any priming of the immune system, and it’s possible for an individual to have an allergic reaction even when they have been exposed for the very first time. The allergic reaction to poison ivy is a good example of type IV hypersensitivity.
Allergy – What it is not
The human body can also produce other reactions that look like allergic symptoms, but the immune system isn’t responsible for these reactions. These can be toxic reactions that might affect everyone with enough exposure. Some examples include bacterial toxins causing food poisoning, toxic reactions due to genetic conditions such as lactose intolerance (caused by lack of a specific enzyme that is required to digest milk sugar), as well as hypersensitivity to things such as gluten that affects people who have Celiac disease. Some symptoms may also be caused by exposure to various medications such as ampicillin and aspirin, MSG (monosodium glutamate – a popular flavor enhancer in foods), various food dyes as well as some psychological triggers. A person suffering from these conditions and diseases should be checked by a health practitioner. Still, it is important to understand that these reactions are not allergies, and allergy testing will not identify these conditions.
Symptoms and Signs of Allergies
Two types of hypersensitivities are typically known as allergies. As you must know, these are type I and type IV. Type I is the name given to immediate hypersensitivities, and type IV is for delayed hypersensitivities.
Signs and Symptoms of Type I Allergic Reactions
A severe type I allergic reaction may lead to itching, dermatitis, and hives on the skin. In contrast, it may lead to eczema and atopic dermatitis over time.
An acute allergic reaction may cause throat tightness, sneezing, nasal congestion, coughing in the respiratory tract, and over time, may lead to asthma. It might also lead to red and itchy eyes.
If there is an allergic reaction in the digestive tract, it usually begins in the mouth with a feeling of a metallic taste, itching, and tingling feeling with throat and tongue swelling. It is typically followed by vomiting, muscle spasms, abdominal pain, as well as diarrhea. Over some time, it might lead to several problems in the digestive tract.
As far as the severity of type I allergic reactions is concerned, it varies a lot. Some symptoms can be mild and short-lived, but there can also be some serious and life-threatening reactions. An acute allergic reaction can be life-threatening as it might cause anaphylaxis, which is the name given to a multi-organ allergic reaction. It might begin with an agitating feeling and a feeling of impending doom.
Some of the common symptoms include fainting and low blood pressure causing pale skin. Some of the other common symptoms and signs of anaphylaxis are cramping, diarrhea, vomiting, chest tightness, trouble in breathing, wheezing, difficulty in swallowing, swollen throat, hives, and red rashes. An adrenaline or epinephrine injection should be administered quickly to prevent anaphylaxis from turning fatal.
Signs and Symptoms of Type IV Allergic Reactions
Delayed allergic reactions are categorized as type IV hypersensitivities, and these are usually skin reactions. One typical example is an allergic reaction to nickel present in metal jewelry. This type of hypersensitivity may lead to skin hardening, swelling, redness, skin inflammation as well as rashes at the place of exposure, and the reaction might surface after many hours or days after the exposure.
Tests for Allergies
The testing begins with the careful review of the individual’s symptoms for diagnosing type I hypersensitivities. This review also includes personal history, family history, and other things such as the age at which the hypersensitivity began, seasonal symptoms, symptoms after exposure to dust, hay, or certain animals or symptoms that appear only in particular environments such as work or home. Various other lifestyle and environmental factors, including exercise, drugs, alcohol, smoking, pollutants as well as stress, should also be considered as these may worsen symptoms. Specific testing for identifying the triggers can be done after narrowing down the possible list of allergens.
What’s Included in Laboratory Testing?
Blood Testing for Identifying Allergen Specific IgE
A blood test is used for the diagnosis of allergies. In this blood test, allergen specific IgE antibodies are measured as it helps in identifying allergy to a substance. In this test, a blood sample is taken, and the sample is tested against each suspected allergen. Sometimes, allergens are chosen individually, but at other times, panels are chosen, such as food panels that typically include the most common child or adult food allergens. Similarly, there are also regional grass and weed panels that typically include the most common airborne allergens in the area where the person being tested lives.
Individual allergen selection is very specific, and common examples include egg yolk vs. egg white, honeybee vs. bumblebee, and western ragweed vs. common ragweed. The health practitioner chooses the most appropriate allergens for testing. In most cases, an individual is truly allergic to only a few substances (usually restricted to 4 or less). If the results of an IgE test are negative, it means that the individual is unlikely to be hypersensitive to that substance. Still, it is recommended to perform a positive test, considering the clinical history of the individual. There are cases wherein an individual can severely react to an allergen even when they have a low level of IgE, and it is also possible for someone to never react even after having an elevated level of IgE. Children sometimes outgrow a food allergy, but they continue to show positive test results for IgE, for several years.
Blood testing for allergy is traditionally done with the method known as RAST, which stands for radioallergosorbent test. This method of testing has been largely replaced in favor of the new IgE specific immunoassay methods. However, some health practitioners tend to call even the newer IgE allergy blood tests as RAST even when the methodology used is different.
The tests for an ongoing allergic process are done through total IgE testing. In this blood test, the total amount of IgE protein that includes allergy antibodies is detected. Still, it is not used to identify particular allergens. The IgE level might be high due to other conditions besides allergies.
Difference between WBC and Complete Blood Count
In this test, the total amount of eosinophil (a type of white blood cell) is detected. A person with allergies might have a higher level of eosinophil.
The health practitioner might also utilize tryptase and/or histamine blood tests for the diagnosis of mast cell activation or anaphylaxis.
Allergy Tests – Other Types
- Scratch or Skin Prick Tests – These are usually performed in the office of dermatologist or allergist, and these tests need to be performed by a trained professional. These tests are used for the detection of various airborne allergens, including mold, dust, and pollen. The skin prick tests are typically not used for determining food allergies, as these have the potential to cause a severe reaction. The individual being tested shouldn’t have significant eczema, and they shouldn’t be taking certain antidepressants or antihistamines for several days before the date of the skin prick test. This test may give false positives for a non-allergic person in case the allergen dosage is high enough.
- Intradermal Allergy Skin Tests – These tests are performed with the help of injections that cause the formation of a bubble under the skin. These tests have not received wide acceptance as these have a high false-positive rate.
- Patch Testing – The patch skin tests for delayed hypersensitivity are the easiest method for testing of type IV hypersensitivities. In this test, the suspected allergen is applied in the right concentration. It is applied under a non-absorbent adhesive patch and is left in place for 48 hours. If the person being tested develops itching or burning sensation rapidly, the patch should be removed. In case there is skin swelling or redness with some hardening or a blister-like formation, it indicates a positive test. In some cases, the allergic reactions appear after the removal of patches, and this is the reason the patch area is also checked for symptoms at an interval of 72 hours and 96 hours.
- Oral Food Challenges – These are widely considered the gold standard when it comes to the diagnosis of food allergies. These tests are labor-intensive, and, close medical supervision is required due to the potential of severe reactions that might include potentially fatal anaphylaxis. In these tests, the individual is given potential food allergens in small amounts in intravenous form or in the form of a capsule, and they are kept under constant supervision for any allergic reactions. A negative reaction is confirmed by the consumption of meal-sized portions of the foods being tested.
- Food Elimination – It is another way of testing certain food allergies. In this test, all the suspected food items are eliminated from the diet, and foods are then reintroduced, one at a time, to determine the foods that are the root cause of the problem.
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