in this video I will teach you everything that you need to know about the structure and to some extent the function of an antibody now you've probably seen an image that looks like this you look at this image and you know that this is an antibody but there are so many different parts of the antibody so many different regions each responsible for different functions that even if you know that this is an antibody that's honestly not enough for USM and comlex so the purpose of this video is to take a deep dive into the nitty-gritty details and simplify all of this pointing out high yield facts along the way so broadly speaking there are two different regions of the antibody the bottom region is the FC fragment and the top region is the Fab fragment the reason that these are called FC and Fab is that the C in FC stands for crystallizable don't even worry about the word crystallizable it's pretty much Irrelevant for the purposes of USMLE and complex the reason that it's faab is AB stands for antigen binding so right off the bat one really high yield topic to know is that the top region the Fab fragment that is the region of the antibody that contains the antigen binding site now in the center of the antibody connecting the top and the bottom region is this kinked area known as the hinge region now just as the name implies and just as the hinge of a door does the same function this allows the anab body to assume various formations and shapes which in short allows it to carry out its immune function better now if we look at the antibody at the top and the bottom we can highlight heavy chains shown here in purple there are heavy chains in the Fab fragment and heavy chains in the f C fragment there are also light chains which I put in this sort of Plum color here the light chains only occur in the Fab fragment they do not occur in the FC fragment now holding this all together are disulfide bonds shown here in Orange there are disulfide bonds connecting the heavy and light chains specifically in our Fab fragment and there are disulfide bonds in the hinge region connecting the right and left side of the antibody now at the top of our Fab fragment between the light and heavy chains we have antigen binding sites this is literally as the name implies where the antigen binds to so there are a lot of different antigens that can be bound in the Fab fragment and part of that has to do with how the genetic makeup of the antibody is coded so before we go any further let's kind of summarize here Fab fragment AB for antigen binding the antigen binding site is between the heavy and light chain in the Fab fragment there is an FC fragment which stands for crystallizable but I want you to ignore crystallizable I don't care about that word that has only heavy chains holding this all together in the hinge region are some disulfide bonds now let's take a look at our two regions because what shows up most often on usmon comlex are some unique features of these regions let's start with the FC fragment now I told you I don't care about the word crystallizable the c in FC does stand for crystallizable but moving forward you can forget that word instead when you think of FC I want you to think of the following c-words C for constant C for complement C for cells bind here and C for controlling the isotype so what does all of this mean the FC fragment is constant whereas as you see shortly the Fab fragment is variable so this is a constant region it doesn't change it fixes complement so the antibody depending on the type of antibody is involved in fixing the complement Cascade cells bind here meaning that this region can be bound by macroasia and mass cells and lastly and probably most importantly it controls the isotype and isotype is a fancy way of saying what type of antibody are we talking about for example IGM IG IGA Etc that's called the isotype and this region is mostly responsible for controlling the isotype now let's contrast that with the faab fragment our top region now I already told you that AB stands for antigen binding but some other things you want to know with AB is that this is very I know that sounded weird as I said it it's variable the bottom region the FC region is constant C for constant but AB the ab in variable so this is a variable region of the antibody its genetic composition can change and that's really important because you have to be able to change the genetic composition of this area of the antibody in order to recognize a wider array or a wider breath of potential antigens and lastly the ab region is responsible for deter determining the idiotype of the antibody now admittedly this definition is a little bit redundant but idiotype refers to the antibodies ability to bind to various different antigens so if you memorize AB as antigen binding site you memorize AB as very and you know that it has to be variable in order to bind different antigens those two definitions put together is really what idiotype refers to but when you're studying for USMLE comx and in-class exams just understand that idiotype refers to the variable binding of different antigens in the top an isotype refers to the actual type of antibody at the bottom idiotype versus isotype very very important so now let's come back and look at both of our different regions of the antibody again faab at the top and FC at the bottom one very important thing to understand for USMLE and comlex is that there are different regions of all of our different heavy and light chains so these can be separated into constant or variable regions and what you'll note is that the two tips meaning the two ends of our Fab fragment that contain the antigen binding site those are variable regions meaning that there is different types of genetic combinations that can code for different antigen recognitions so the variability of an antibody occurs at the tips of the faab region in your heavy and light chain and that should make some sense to you again if that's the part of the antibody that binds to the antigen then there is an advantage to having that be variable meaning that its composition can change so that it has lots of different recognition capabilities however if you look at the heavy chains in the FC fragment and the two bottommost portions of both the heavy and light chain in the Fab fragment you will note that these have C's meaning constant these do not change and we can see that there's either an H or an L after all of these letters H stands for heavy and L stands for light the highest yeld thing to know for USMLE and complex is that test writers like to ask you which region of the antibody is responsible for which function and so a classic test question involves numbering different areas of the antibody for example as you see here now I've only drawn three numbers in but you really could have drawn numbers going up to six and then what the test writer will do is they will ask you which combination of numbers or which singular number is responsible for some function the highest yield question that I can think of is shown on the slide they'll number various regions of the light and heavy chain and ask you which part of the antibody is responsible for determining the isotype now there are two different answers to this question and one is better than the other so if you have to pick between just the fragments meaning FC fragment versus faab fragment then the answer would be whatever elements are numbered in the FC fragment however if one of the answer choices includes a combination of the regions that you see on my slide numbered as one 2 and three which is the constant heavy regions that is the most correct answer and so I know lots of you are probably sitting at home and you're like whoa whoa whoa dirty hey it's my understanding that the FC fragment determines the isotype and you would be correct however it is very important to understand that the constant heavy chain which technically is in the Fab fragment also contributes to isotype determination because it's constant it's not variable and only in the heavy chain right not constant light chain so not CL that's a very very high old question and that's why I put this specific question in my presentation because people get tripped up on this all the time A lot of times they'll incorrectly select the answer choice that says regions two and three when the most correct answer is Regions one 2 and three so that is the most important thing to know specific to this question but in general when it comes to antibody structure and function know what isotype means know how isotype is determined know that isotype is determined by these three regions know what idiotype means know which part of the antibody contains the antigen binding sight and understand in general the difference between light and heavy chains variable versus constant and the presence of disulfide bonds that is antibody structure and antibody function the next video will'll go into the in-depth differences between the various isotypes I.E IG versus IG versus IG versus IG etc etc