High Yield Notes
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Type I hypersensitivity
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Type I hypersensitivity

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Type I hypersensitivity reactions are mediated by antibodies.

Questions

USMLE® Step 1 style questions USMLE

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A 6-year-old girl comes to the emergency department because of acute onset of urticaria, dizziness, and shortness of breath and stridor after eating peanut butter 30 minutes ago. On arrival her temperature is 37°C (98.6°F), pulse is 120/min, respirations are 20/min, and blood pressure is 120/90 mm Hg. Physical examination reveals diffuse raised red lesions with mild angioedema to the tongue and laryngeal edema resulting in stridor.  She is immediately treated with an epinephrine injection and her symptoms quickly resolve. Which of the following molecules is most associated with this patient’s condition reaction?

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Transcript

Having a hypersensitivity means that someone’s immune system has reacted to something in such a way that it ends up damaging them, as opposed to protecting them.

There are four different types of hypersensitivities, and in the first type or type one, the reactions rely on Immunoglobulin E, or IgE antibody, which is a specific type of antibody - the other major ones being IgG, IgA, IgM, and IgD.

So because IgE is involved with type one hypersensitivity reactions they are also called IgE-mediated hypersensitivities.

This type of reaction is also sometimes called immediate hypersensitivities, because the reaction happens super fast—on the order of minutes.

So most allergic reactions are IgE-mediated, and therefore most allergies are type I hypersensitivity reactions.

“Allergy” comes from the Greek Allos which roughly means “other” and ergon which means “reactivity”.

Essentially, allergies are reactions to molecules from outside your own body that most people don’t react to—and these are specific molecules from things you might breathe or take in like foods, animal dander, bee stings, mold, drugs or medications, and pollen.

You can also mount an allergic reaction to things you come in contact with on your skin like latex, lotions, and soaps.

These specific molecules are also called antigens, and when they cause an allergic reaction, they’re called allergens.

An allergic reaction happens in two steps, a first exposure, or sensitization, and then a subsequent exposure, which is when it gets a lot more serious.

People that react to these allergens usually have a genetic predisposition to having over-reactions to unknown molecules or allergens.

This means that these people have certain genes that cause their T-helper cells to be more hypersensitive to certain antigens.

Since the production of these T-helper cells is genetically linked, allergies to things tend to run in families.

So let’s say this person breathes in some ragweed pollen, that person happens to have T-helper cells that can bind to a specific molecule on the pollen, making that molecule an allergen.

First off, that antigen gets picked up by immune cells hanging out in the membranes along the airways, which then grab the molecule and migrate to the lymph nodes, which happens regardless of if the person is allergic or not.

These cells are antigen-presenting cells, since they carry the antigen to the lymph nodes and present it to the T-helper cells living there.

Dendritic cells and macrophages are examples of antigen-presenting cells.

When the person is allergic, the antigen presenting cell will also express costimulatory molecules, which are needed to mount an effective immune response.

Before the T-helper cell sees the antigen though, it’s called a naive T-helper, since, even though it’s built to recognize the antigen, it hasn’t actually seen it before.

When the T-helper gets its hands on the antigen though, and also binds the costimulatory molecule, it’s now been primed, and the naive T-helper changes into a different sort of T-helper cell.

Usually in type I hypersensitivity it differentiates into a type 2 T helper cell, or just TH2 cell, and this step happens in response to various small proteins or interleukins that might be floating around at the time.

Some interleukins that sway the T-helper cell into turning into a TH2 cell are interleukin 4, interleukin 5, and interleukin 10, and these are all cytokines - and they’re given numbers because it’s easier to keep track of them that way.

At any rate, the excited TH2 cells release a bit of their own interleukin 4 and get the B cells to undergo antibody class-switching, and so the B cell switches from making IgM antibodies to making IgE antibodies which are specific to ragweed pollen in our example.