Fundamentals 1: 11:00 - 12:00 Scribe: Lauren Paul

Fundamentals 1: 11:00 - 12:00 Scribe: Lauren Paul Friday, September 25, 2009 Proof: Kristina Hixson Dr. Burrows Antigen-Antibody Reactions Page 1 of 5 I. Antigen-Antibody Reactions [S1]: II. Learning Objectives [S2] a. We’re going to talk about interactions between antigens and antibodies. What makes a good and a weak antigen. b. Well talk about both in vivo and in vitro. III. Structure of an IgG antibody [S3] a. We just looked at our antibody structure. b. Red part is the antigen binding part—variable region. c. Blue part is the constant region. IV. Antigen-Antibody Binding [S4] a. There’s been a lot of actual structures of antibodies and antigens discovered. b. Just looking at these three pictures you can see that antibodies are able to recognize multiple shapes on molecules. c. Here we have a round ball dropping into antigen binding part of antibody molecule. Here’s a situation where the epitope of the antigen looks like a hot dog. Here we have a large epitope covering the entire surface of the CDR3 region of the molecule. d. They can recognize just about anything. V. Features of the Antigen-Antibody Interaction [S5] a. Features of this interaction are pretty typical of any protein-protein interaction. i. Reversible interaction—antigen and antibody can come together and come apart. ii. Non-covalent—no covalent bonds between the molecules. VI. Non-covalent binding [S6] a. Some of the forces involved in protein interactions are listed here: i. Electrostatic forces—positively and negatively charged AAs interacting quite strongly. ii. Hydrogen bonds iii. Van der Waals forces iv. Hydrophobic forces—AAs that like to interact with each other and they don’t like interacting with water. b. H-bonds and Van der Waals are most important for antigen-antibody interactions. They hold them together. c. The antibody is folded up into the immunoglobulin domains and all the hydrophobic AAs are inside the domain and same with electrostatic forces. VII. Features of the Antigen-Antibody Interaction [S7] a. When we talk about interactions between molecules, they have affinity—the strength of binding between the two molecules; the ease of association and dissociation. High affinity—doesn’t come off easily. b. Avidity—have at least bivalent antibodies, so even if there is weak affinity, we have avidity which is summation of two antigen binding arms. It’s basically the summation of multiple affinities. c. For example, IgM have very low affinity but high avidity because they have 10 arms. They are always coming off but there’s always another arm around ready to latch on to the epitope. VIII. Antigen-Antibody Binding [S8] a. Same picture as before except the third cartoon is different (this is what he’s referring to this whole time). b. We have an epitope binding to an antibody and taking up the whole CDR1 through CDR3 surfaces. You can also see that if the epitope was half of what it once was, the affinity would be less because there’s less surface area per contacts mediated by H-bonds that can stabilize an interaction between the antibody and epitope. IX. Terminology[S9] a. Antigen is a very old term and originally meant able to generate antibody. Now it’s defined as a substance that can be recognized by antibody or T cells b. Immunogen is a substance that can actually itself generate an antibody response. c. Usually synonymous terms and used interchangeably but sometimes they aren’t which we’ll talk about in a minute. X. What makes a good antigen[S10] a. Size—large molecules are more immunogenic so they generate better antibodies. b. Chemical composition—proteins more immunogenic because they have complex 3D shapes with bumps and grooves as opposed to carbohydrates which tend to be repetitious (or lipids). c. Similarity to self-antigens—so if we took someone’s albumin and immunized their neighbor with it, they probably won’t make any response, say there was one AA difference. If we took your albumin and immunized a chicken, where there’s a large difference, there would be a large response. Important because we obviously don’t want to make antibodies to our own bodies and sometimes we do and we get auto-immune diseases. So therefore the further distant and less similar antigens will produce a larger, more powerful antibody response. d. The way you give the antigen have some affect on whether its immunogenic. Usually if you’re getting immunized with something you can get an IM or SubQ intermuscular. Oral immunization is weak. Fundamentals 1: 11:00 - 12:00 Scribe: Lauren Paul Friday, September 25, 2009 Proof: Kristina Hixson Dr. Burrows Antigen-Antibody Reactions Page 2 of 5 e. Adjuvants—non-specifically prime up and boost up the immune response. Most vaccines you get, like tetanus, are actually not just tetanus toxide protein; they are complexed with almunimum hydroxide or some other almunimum salt called alum (adjuvant for humans). Stabilizes the protein so it stays around longer and may also stimulate some other receptors that prime up the immune system. f. MHC—we’ll talk about that next week. g. But essentially what makes something immunogenic is it’s complexity—if it has a lot of epitopes you can stimulate a lot of B cells and make a lot of different antibodies, and it’s foreignness—the further distant it is from yours, there are more epitopes you aren’t tolerant to. h. Next week we’ll talk about T cell responses. XI. Some well-known antigens [S11] a.

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