And now we turn to immunoglobulin receptors. Immunoglobulin receptors are basically just antibodies that have been stuck in the membrane. And they are there to transmit information from the outside of the cell to the interior. So what we're going to look at right now is the structure of the receptor, it's associated signaling molecules, and a wee bit of an intro on how the whole system activates and transmits information. Here we have two different antibodies, one with a rigid bend could be an M, could be an E. One with a flexible hinge, that could be a D or an A, or a G. Antibodies are soluble, they're found in the blood plasma, the lymph, and the interstitial fluid. If we're looking at an immunoglobulin receptor however, we can see it looks much the same as an antibody but it's stuck in the plasma membrane. And it's sticking out into the exterior of the cell. So here's one for the D on the right. One for the M on the left. And if you have an IEV cell, you can find both of these types embedded in the membrane and moving around over the surface of the cell. They can slide around, but they can't pop out because, of course, they have this little button like structure down in here. And they can't enter the cell, because of course, they can't reach through the membrane with their immunoglobulin large groups. Now if you look at the membrane spanning regions, you can see that they're on the C terminal of the heavy chain. They're synthesized last, therefore during protein synthesis on the ribosome. And they're going to end in this very small structure that's going to keep them from leaving the membrane. When B cells recognize foreign antigens, they recognize it when two neighboring receptors will bind to that antigen and cross link. That will begin the recognition process, but you can see that they themselves cannot begin the signalling process, because their cytoplasmic regions are very, very short and don't contain any signalling domains. That is the job of the next pair of molecules, which are the co-receptors or sometimes called the co-signaling molecules. You can see we have a heterodimer here, and it is linked by a disulfide linkage, so it's stuck together. And we'll see it has long tails with signalling domains in the interior, or in the cytosol of the cell. These were originally called the immunoglobulin alphas and immunoglobulin betas. Lately, they have changed the name to CD79A and B. And basically, the tails have a region that can pick up a covalent phosphate. These yellow regions are called ITAMs for immuno tyrosine activation motifs. And the tyrosine refers to the fact that the phosphates will be put onto a tyrosine. And that will result in the activation of the system. So here we have the M class immunoglobulin receptor complex, and so whereas our M class receptor with its alpha betas. And here we have the corresponding version for the D class receptor. And this is what you will find, essentially sliding around the membrane of the naive B cell after it is released from the bone marrow. It is, of course, out there trying to find a foreign antigen. And so, let's pretend we've got a foreign antigen, and that, that foreign antigen just happens to be something that the recognition regions of these immunoglobulin receptors can bind to. So both of these things are moving around, and by dumb luck, our M class receptor hooks up to this antigen. And later on, when the D class moves around and happens to hit it. They will both stick to that antigen, and that antigen will cross link the two receptors. When that happens, the signalling molecules will be brought into contact with each other and kind of bang around, and that will change their confirmation. When their confirmation changes, the ITAMs will pick up phosphates. That is, proteins will add phosphates covalently to the parts of these that are extending into the cytoplasm. And those parts will form a docking site for signaling proteins to bind and begin the signaling pathway to the interior, and that's something we'll take up later on in a later lecture. Now let's hope we made lots of antibodies. And our immune system has conquered whatever hideous viral infection we've got, and we're now going to make some memory cells. We can actually make memory cells for all the different classes of antibodies. That is, we can make memory cells to all four different G classes, both A classes, and the E class. And these memory cells will lie in wait in case that antigen shows up again. Here I pictured a cell with a lot of IG3 immunoglobulin receptor complexes. And I want to point out that once you have class switch away from the naive cells which can make MND. From then on in, whatever class of antibody you are producing, that is the only class of receptor you will have in the surface of your cell. So in this case, we're going to this rather interesting looking one with the long, flexible neck like region. Note that it has the same co-signaling molecules that the M and D cells had. Once we've decided to make something other than an M and D class naive cell receptor complex, any of the membrane receptors that you have on the surface will all be identical to each other both in their TEM regions and in their recognition sites. Another thing I'd like to remind you is Is that if you see insoluble M class antibodies, they're in a group of five. The ones in the membrane, the membrane receptors are always singles. That's also true of A class antibodies, which can be found in groups of two or even three, if they're soluble. But when an A class antibody becomes an immunoglobulin receptor, then one and only one unit will be in the membrane and it will be associated with these alpha beta core receptors as shown here.
