Mechanisms of Acquired Immunity

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Mechanisms of Acquired Immunity

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 32 Current Lecture Immunoglobulins If you take serum proteins, they are all globular proteins – so they are called globlulins. And if you separate these globulins by electrophoresis, you come up with five classes. First class, while globular, is not called a globulin. It is the A class – A stands for Albumin, the most abundant serum protein. The other 4 classes are called globulins: alpha1, alpha2, beta, and gamma. The gamma globulins are antibodies or immunoglobulins. There are 5 classes of immunoglobulins: these are all protein secreted by B-lymphocytes. The most abundant in the plasma is IgG, which is shown in the box. It has four parts, and two pairs of identical subunits – two pairs of light chains and two pairs of heavy chains joined by disulfide bridges. Has a Y shaped structure. There are constant C and variable V regions. The places that bind the antigen are the tips of the variable regions. The heavy chain has a hinge region, which is flexible. That allows the angle between the two antigen binding sites to vary and move. Fab fragments occur when you take antibodies and you digest them with a proteolytic enzyme called papaine. It cleaves to make two fragments, the antigen binding fragment (Fab) and the crystallizable fragment (Fc). Called the crystallizable fragment because these Fc’s are made up largely of a chunk of the constant region, which is pretty constant between antibodies, so you can put all these Fc’s together and crystallize them. All of these are secretions of plasma cells, or activated B lymphocytes. How do plasma cells get activated? At the top, you can see IgD. And its structure looks like the thing in the box, but there is a transmembrane region in the constant region of IgD’s. So IgD is a membrane protein. It is anchored by the transmembrane region, and an IgD is the B cell receptor. When an antigen binds to the antigen bindin region of that IgD, this B cell will get activated. When the B cell is activated, it proliferates to form plasma cells and memory cells. The plasma cells will secrete antibodies, and the first ones they secrete will be IgM, which is a huge molecule consisting of 5 subunits IgD held together by disulfide bonds and a joining chain that connects a couple of them. It is too large to leak out of capillaries. Even in inflamed capillaries, which are much more leaky, IgM does not leak out. IgM’s arena action is mainly in the blood. So after about a month of a plasma cell secreting IgM, it begins to undergo class switching. And what happens is that the gene for the constant region is switched form the type that is specific for IgM to the type that is specific for IgG First class switch is from IgM to IgG. The same plasma cell that was secreting IgM now secretes IgG. The class switching was in the constant region, and not ...

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