 All right, we're bringing this lecture around full circle. We started out with this whole, like the whole point of this was talking about exchange, and capillary exchange. And one of the things that we talked about in capillary exchange was the presence of these plasma proteins in the blood, yes? And tell me, what did the plasma proteins, what did they have to do with? Why did we talk about plasma proteins? Plasma proteins provide the osmotic pressure that brings fluid back into the capillary so that you don't end up like a blueberry, like Vila Beauregard, boom. Okay, thank you for that. Think about the process we just talked about. Think about hemostasis. Are there any plasma proteins involved there? All those agents and inactivated, the liver is producing fibrinogen and prothrombin, and those are both important to clotting and they're plasma proteins that are floating around in your system. This little brief clip is about antibodies. Antibodies are also plasma proteins that are floating around in your system, and they are creating that osmotic pressure. Antibodies are unique proteins. They all have the exact same shape, except for this little part that I showed you in green. I'm going to make another one over here, and this one I'm going to make the little part look a little different in blue. These are the variable regions of an antibody and the variable regions match up to an antigen. Now, check this out. Antibodies are plasma proteins that are involved in the immune response. They have a variable region that matches to an antigen, so it matches an antigen. And we have to define what is an antigen. My friend, my friendies, an antigen is any substance that can possibly initiate an immune response. Oh my gosh, my handwriting is so terrible. Response. This is just unacceptable. Just kidding. It's all crazy. Anything that could possibly initiate an immune response is considered an antigen. Sometimes... Okay, hold on a second. All of your cells are marked with antigens. So look, an antigen is just like a molecule that is hanging off of your cells. And oh, that's what mine looks like. Those are my antigens. Those antigens could possibly initiate an immune response. Their proteins embedded in the cells, hopefully in my own body they don't initiate an immune response. Hopefully my body's like, dude, that's me. Look at that cute little heart. That's me, of course that's me. Only I would make cells that look like that and my immune system won't attack them. It won't initiate an immune response because my immune system recognizes that as self. But we all know that there are conditions where that's not working properly. And if it isn't working properly, then you have an autoimmune response where your immune system does attack self cells, which is obviously not a good thing. Okay, so the antibody is the thing that is floating around in your blood. It's the protein that's floating around in your blood that can bind to the antigen and possibly initiate the immune response. So it's the thing that's like helping to do the monitoring. The variable region of the antibody matches to an antigen and they can have like crazy numbers of possible variable, like crazy numbers. And they're very specific. They will match to a specific pathogen or a specific molecule on a pathogen and initiate an immune response through the strategies that antibodies have. I'm telling you this because we're going to talk in the next section about blood typing, which again that's the last piece in our whole little blood adventure. And blood typing has to do with particular antigens that are on your red blood cells and antibodies that are found in your plasma. And the antigen and the antibody determines like who you can get an organ transplant, a blood transfusion from. So know that antibodies initiate immune responses. I mean they respond to a bad guy coming in. Know that antigens are embedded in all of our cells and we shouldn't react to things that are our own. We shouldn't have antibodies against self antigens. We should have antibodies against non-self antigens and blood typing has to do with this. Okay, so let's talk about our blood types.