Question about a quiz 2 question

Hi everyone! Here’s a good question I got by email – thought I’d share it with you in case someone else had the same question.

Q. Hello, I was wondering if you could send me the question that had the answer “Type 2” hypersensitivity. I got it wrong and I think the reason behind it was because of the “platelet” term and I took it maybe too literal by thinking that in the illustration of Type 3 in your notes where there is platelet aggregation in the picture. I also factored Type 2 out because it only mentioned red blood cells under type 2 which I took maybe more literal once again because I didn’t see anything about platelets.

A. Yes! I’ll explain why type II hypersensitivity is the correct answer for that question…and then I’ll answer your questions specifically. Here’s the question you’re referring to from quiz 2 (I also posted a link to the Quiz 2 Kahoot on our Kahoots page):

This patient is making antibodies against her own platelets, and those antibodies are coating (opsonizing) her platelets, making the platelets yummy to macrophages. This is an example of type II hypersensitivity, because there are antibodies coating a cell, leading to problems (in this case, consumption by macrophages). Here’s the slide from lecture showing how this process works:

This is exactly what’s going on in the patient in this quiz question – you could just cross out “opsonized cell” in the diagram above and put “platelet” and that would be a diagram specific for this patient.

I hope that makes sense! Just for completeness, here’s your email, with my responses to your questions in bold:

Hello, I was wondering if you could send me the question that had the answer “Type 2” hypersensitivity. I got it wrong and I think the reason behind it was because of the “platelet” term and I took it maybe too literal by thinking that in the illustration of Type 3 in your notes where there is platelet aggregation in the picture. You’re right – there is platelet aggregation in the picture illustrating type III hypersensitivity! But in type III hypersensitivity, the patient has immune complexes floating around, lodging in vessels, and the platelets are there because of all the damage to the vascular endothelium. The platelets are kind of a secondary thing – they’re not coated with antibodies, or anything – they’re just showing up because of tissue damage. The patient in our quiz question, on the other hand, doesn’t have immune complexes – she has platelets that are coated with antibodies! Here, the platelets aren’t just showing up because there is damage – they are actually being targeted by antibodies – so their role in this patient is very different. I also factored Type 2 out because it only mentioned red blood cells under type 2 which I took maybe more literal once again because I didn’t see anything about platelets. Oh – actually type II hypersensitivity reactions aren’t limited to just red cells! They can target any cell – red cells, thyroid cells, platelets, neurons – literally any cell in the body!

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