Hypersensitivity Type I

Last week, we did sort of a brief overview of the hypersensitivities and their types. This week, we’re going to start with number 1!..or really, number I…

We usually think of type I hypersensitivies as immediate or allergies. It is mediated by IgE, which makes sense because that is the first responder of gammaglobulins, right? Typically, this response is fast, within minutes. When you think of someone having an allergice reaction, we don’t normally think of them being around a cat or a dog and then getting short of breath a few days later, right? They are usually like “something is happening and it is happening right now.” It is also important to note that the individual would have been previously exposed to whatever antigen that we’re using as an allergen.

There are basically two phases in a type I hypersensitivity, immediate reaction and late phase reaction.

The immediate reaction, obviously, happens right away. This is the one that is within minutes. During this phase, there is a local vasodilation, an increase in vascular permeability and in vascular leakage. Any cytokines or mediators coming to mind here? Yup, histamine. And what do drugs do people take to treat their hay fever? Anti-histamines.

The late phase reaction takes hours to occur, up to a day. Now, we’re looking at tissue damage and leukocyte infiltration. These include eosinophls, neutrophils, basophils, monocytes and helper T cells, so damn near the whole army shows up.

So what happens that this will occur? The body isn’t allergic to something on the first presentation. Instead, the allergen is given to a CD4 helper tell cell, which then secretes some cytokines: IL-4, IL-5 and IL-13. The most important one here is the IL-4. It’s going to act on B-cells to promote class switching to IgE. IL-13 is then going to enhance IgE production. Now, reexposure to that allergen leads to crosslinking of IgE molecules, which leads to the pre-sensitized basophils and mast cells kicking in. The fact that they’re presynthesized is part of why that reaction can happen so incredibly quickly.

Of course, it does get deeper than that, but I think this is enough to understand the general concept of hypersenstivity type I. Essentially, think allergies or allergic response. This is the underlying pathogensis for allergic asthma, conjunctivitis, anaphylaxis and even food allergies.

Stick around for next week, and we’ll look at type II!

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