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13.6.2. Hypersensitivity

Hypersensitivity. Allergies

The function of the immune system is to destroy foreign antigens without causing any harm to the person, but, on occasions, these responses can be of excessive force in the face of harmless or not very dangerous molecules, and can produce serious effects on people, even death.

Two types of hypersensitivity are distinguished:

Immediate or anaphylactic hypersensitivity: allergy

The allergy is a hypersensitivity mechanism or exaggerated immune response to the presence of antigens safe or very dangerous.

The immediate or anaphylactic hypersensitivity develops rapidly, about 10 to 20 minutes after exposure to the antigen, which in this case is called allergen.

An allergen is a substance that induces a hypersensitivity (allergic) reaction in susceptible people who have previously been in contact with them. Some of the most common allergens are bee venom, pollen proteins, animal hair, dust mite feces, some medications like penicillin, some foods like shellfish, peanuts, etc.

Development of the allergic reaction

The allergic reaction develops following these phases:

1.- Allergen uptake by antigen presenting cells.

2.- Sensitization to the allergen.

When the first contact with the allergen occurs, no external symptoms are shown, but the immune system recognizes the allergen since the antigen-presenting cells (APCs) phagocytize it and show its fragments in the MHC-II to the helper T lymphocytes (TH), which recognize and activate neighboring B lymphocytes. The B lymphocytes into plasma cells and release large amounts of IgE immunoglobulins which bind to the constant region of basophils blood and mast cells tissues. This is the process of awareness raising.

3.- Release of the content of the histamine-filled mast cell granules.

When the second contact with the allergen occurs, the allergic reaction occurs. The allergen binds to the IgE that coat mast cells and basophils, causing their degranulation, expelling substances from the cell cytoplasm. These secreted substances cause inflammation (histaminesprostaglandins, serotonin, etc.) and allergy discomfort, such as dermatitis or diarrhea, if it is a food allergy, mucus secretion if it is an allergy caused by substances contained in the air, etc. If mast cells and basophils discharge histamineblood, dilation will causes blood vessels, leading to a generalized reaction, with the contraction of the bronchioles and generally vasodilation can cause death by asphyxiation or a significant decrease in the blood pressure (anaphylactic shock). 

Delayed hypersensitivity

It can also happen that the reaction occurs later, even several weeks after contact with the antigen, as in delayed hypersensitivity.

Antibodies are not involved in delayed hypersensitivity, but rather a type of T lymphocytes that, after a second contact with the same antigen, release substances that stimulate the action of macrophages and trigger an inflammatory process.

An example of this type of hypersensitivity is the cause of contact dermatitis caused by some cosmetics, clothing, jewelry or plants in contact with the skin, whose proteins bind with haptens present in these products and originate the antigens that trigger the disease. reaction. Since the symptoms (itching, rash, ...) are similar to those of anaphylactic hypersensitivity, these reactions are incorrectly called contact allergies.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Aragon, September 2019, option B, question 5.

Answer the following questions: (1 point)

a) Define: autoimmune disease and allergic (or hypersensitivity) reaction. (0.5 points)

b) Define passive natural immunity and give an example. (0.5 points)

Aragon, June 2018, option B, question 1

Genetics and environmental factors play an important role in the origin of allergies. (1 point)

a) What do we call an allergy? (0.5 points)

b) What does the allergic response produce? (0.25 points)

c) What is the nature of antigens and antibodies? (0.25 points)

Aragon. June 2017, option A, question 5.

Define the following concepts related to the immune system: hypersensitivity. (0.2 points)

Aragon. September 2015, option B, question 5.

Define the following statements: (2 points)

Hypersensitivity reaction. (0.5 points)

Aragon. September 2014, option B, question 4 .

Define the following terms referring to immunity: (2 points)

Allergic or hypersensitivity reaction. (0.5 points)

Andalusia, September 2014, option A, question 5.

Pollen is a foreign body for the body that, in allergy sufferers, causes annoying reactions (cough, sneeze, itching, nasal congestion).

Why a first contact with pollen in these people may not cause external symptoms, which do appear after a second exposure? [0.5].

Does this mean that those allergic to pollen are immunodeficient? [0.5]. Reason for the answers.

Basque Country, July 2020, option 3A

It is estimated that in developed countries a high percentage of the population suffers from some type of allergy.

a) (1 point) Indicate what an allergen is. Give some examples of allergens. Give reasons for your answer.

b) (1 point) What type of immune system reaction occurs in an allergy? In this situation, what three basic processes can be triggered?

c) (0.5 points) Mention one type of cell and one type of molecule involved in allergic processes.

Madrid, July 2021, question A5

In relation to the immune response:

Peanuts are one of the most common causes of food allergies. The hypersensitivity reaction appears within a few minutes of ingestion of this dried fruit.

a) What is a hypersensitivity reaction? What is the difference between antigen and allergen? (1 point).
b) Describe the processes that occur during an immediate hypersensitivity reaction using the following terms in the proper order: mast cell, allergen, histamine, IgE (1 point).

Galicia, July 2021, question 8d

Briefly: what are hypersensitivity and autoimmunity?


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