hypersensitivity has four different types and they each have their own unique mechanism or pathophysiology and they each have their own unique examples and all of this stuff is pretty high-yield hypersensitivity does show up a lot on USMLE and comlex so with that said we're gonna give you a pneumonic that's gonna help you work through all of this in today's video we're gonna go one at a time through each of these four types paying attention to the normal pathophysiology right I'll always explain the normal science and then I'll give you the pneumonic kind of overlaid on top of that so that you can easily learn this in this short video so for just as a summary here's everything that you need to know if you didn't watch this video this is what is in first day this is what's in all the resources out there there are four different types there's type 1 type 2 type 3 and type 4 the mnemonic that you're gonna use today is acid and the a goes with type 1 C with type 2 I with type 3 and D with type 4 so just like there's four types each of those letters corresponds to one of these types of hypersensitivity so let's go through these one at a time I'm gonna explain the normal path ofa's and I'm gonna overlay the pneumonic at the end so that you better recall all of this information so why don't we get started with type 1 hypersensitivity so this one is referred to as allergic hypersensitivity basically what happens is that first and antigen binds to IgE then I GE and that allergen get cross-linked at the receptor this cross-linking occurs at the FC receptor you don't need to know that for us Emily or comlex but it does get tested if you're using this video to study for your class exam after the cross-linking occurs mast cells and basophils release vasoactive substances and those vasoactive substances they caused massive vasodilation so this makes sense right in the context of an allergic reaction or some type of anaphylaxis you know that the end result is massive vasodilation and that is why we give epinephrine when someone is undergoing anaphylaxis because you have to stop the vasodilation and of course you know that epinephrine causes vasoconstriction so basically one is allergic and if I were to summarize type 1 I would say it's allergic anaphylaxis and allergy everything that begins with the letter A so if you recall from our mnemonic acid type 1 goes to the letter A so acid allergic asthma allergy anaphylaxis basically what you need to know besides the letter A is that you get cross linking with IgE and the allergen so type 1 allergic acid allergy anaphylaxis involving an IgE antibody and mast cells and basophils released vasoactive substances that cause vasodilation all right that's type 1 now we're gonna move on to type 2 so type 2 is the C in our acid pneumonic and what does C stand for cytotoxic so what you get here is that either IgG or IgM bind to an antigen and you're gonna see this on all of these slides you're gonna see what's actually happening in a little picture on the left when IG G or IgM fine to this to the antigen into the cell receptor you get complement activation you also can get natural killer cell activation but that doesn't always happen what you get after that is the death of your self cells so this is a type of hypersensitivity that results in the death of cells that are actually native to the human body so type 2 is cytotoxic and again we're in our acid mnemonic we're at the C because we're type 2 so cytotoxic with the C and acid and what you need to remember is that it's cytotoxic involving IgG or IgM and that it's smooth and we're gonna use the C in smooth and you'll understand why after I explain type 3 so this is really high yield that it's smooth antibody deposition so I'm gonna slow down for a second so far we talked about type 1 and type 2 type 1 was allergic anaphylaxis all the A's write the a in acid and it involved I GE type 2 is cytotoxic right the C in acid and it involves smooth with a C antibody deposition so so far both type 1 and type 2 involve antibodies now we're going to go on to type 3 so type three is the I in our acid mnemonic and what does that stand for immune complex okay so I Gigi or IgM we're talking about the same the same immunoglobulins here they're gonna bind to an antigen and when they bind to the antigen they're gonna form these complexes and the complexes are thick they're like little globules of stuff and that globule or that complex is gonna deposit itself in various tissues you right usually at the basement membrane and it's this deposition of complexes that damage the tissues and cause complement to be activated which then destroys those tissues so in all of the diseases that are type 3 hypersensitivity x' the reason that your self native human tissue is being destroyed is because you're you're accidentally activating complement because there's little complexes stuck in these tissues so going back to our acid mnemonic we're now at the third letter because we're type 3 and the third letter was I for immune complex and what you need to remember here is the I for immune complex but the I in lumpy bumpy or lump I bump I so lumpy bumpy is the is the way that antibody complexes immunoglobulin complexes are deposited in the tissues now let's slow down for one second because this is the highest yield part of this entire video I told you in type 2 that you needed to remember smooth with a c smooth linear deposition contrast that to type 3 it's lumpy bumpy or globular or thick deposition so what does that mean and why does that matter what this means is that there are two different types of renal pathologies that can exist secondary to hypersensitivity reactions you can either have goodpasture's syndrome or post streptococcal glomerulonephritis so let's pay attention to that now goodpasture's syndrome is a type 2 hypersensitivity post strep glomerulonephritis is a type 3 hypersensitivity because in type 2 the antibodies are deposited smooth you get a linear deposition and you see that picture at the top notice how the deposition in the basement membrane is smooth however in type three in Post rep glomerulonephritis you get complexes being deposited right because type three is immune complex mediated and because it's up complex which is a little ball of stuff those little balls are spaced out in the basement membrane so instead of being linear like smooth it's lumpy bumpy and these are buzzwords they are very high yield so if you're taking your exam and they show you a picture like this and they use either linear or lumpy bumpy you already need to train your brain to think good pasture versus post strep glomerulonephritis and type 2 versus type 3 hypersensitivity this is without question the highest yield piece of information in this whole lecture and I think that you can appreciate the difference between the two pictures here one is very smooth and the other is very lumpy bumpy or globular globular if that's even a word but this is high-yield spend some time on this slide the way that you can remember this is that good pasture is two words but psg for post strep glomerulonephritis is three letters and they go with type two and type three but at the end of the day if you remember lumpy bumpy versus smooth you'll understand immune complexes immune complex versus linear and type 2 versus type 3 again very high yield I cannot stress that enough no this slide we're gonna end with type 4 hypersensitivity and type 4 corresponds to the D in our acid mnemonic and what does the D stand for delayed so what's happening here an antigen is gonna sensitize a th one cd8 plus cell and that cell is gonna release site okay cytokines that are going to activate macrophages and those macrophages directly remove target cells right they phagocyte ties the cell and kill the cell this is delayed because it takes time to occur the antigen first has to sensitize your cell and then later on are kind of like a repeat insult so to speak the cell releases the substances that then activate activate macrophages so just know that it's delayed it doesn't happen immediately and again D delayed acid is our mnemonic and what you need to remember here is that it's delayed and this is what's occurring with type one biddies this is what's occurring in a contact dermatitis and this is how you diagnose tuberculosis so the TB test that you get in your skin the reason that you have to come back after you've gotten that little shot under your skin to see if there's any in duration is because it's delayed right it doesn't happen right away so anybody who's gotten a TV shot knows that you don't just get injected under your skin and get it red right there and then rather you got to come back 48 hours later to get it red because it's a delayed reaction so if it's gonna be positive it won't be positive until two days later so that's type 4 hypersensitivity D delayed diabetes dermatitis and diagnosing tuberculosis so here is again the summary of everything that you need to know we talked about the pneumonic we talked about a for allergy C for cytotoxic eye for immune complex and D for delayed eye I tried my best to fit in the the clinical examples with the letters again make sure you understand the difference between good pasture and post repla Maryland if Rytas in that good pasture type 2 is smooth or linear deposition whereas type 3 is immune complex or lumpy bumpy deposition that's super super high yield and that shows up all the time on your exams but know this chart and I hope that I was able to simplify this really complex topic for you guys best of luck