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BIO120H1 (305)
Chapter 4

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University of Toronto St. George
Bebhinn Treanor

Chapter 4  In an adaptive immune response, the body is cleared of extracellular pathogens and toxins by antibodies (secreted form of B cell antigen receptor) o Antibodies are produces by effector B lymphocytes or plasma cells of the immune system in response to infection o Circulate as major component of plasma in blood and lymph and always present at mucosal surface o Antibodies most often recognize proteins and carbohydrates o Binding of antibody to bacterium or virus can disable the pathogen and make it susceptible to destruction by other components of immune system o Antibodies are best source of protective immunity and the most successful vaccines protect through stimulating production of high quality antibodies  An individual antibody molecule is said to be specific o Can bind to only one antigen or small number of different antigens  Antibodies are diverse in their antigen binding specificities o The total number of diff specific antibodies that can be made by an individual=antibody repertoire can be high as 10 . In practice, the 9 number of B cell limits actual repertoire to 10 .  Antibodies are the secreted form of immunoglobulins o During its development, individual B cell becomes committed to producing Ig of a unique antigen specificity; thus mature B cells collectively produce Ig of many diff specificities o Before it encounters antigen, mature B cell only expresses Ig in a membrane bound form that serves as the cells receptors for antigen. o When antigen binds to this receptor, the B cell is stimulated to proliferate and to differentiate into plasma cells, which then secrete a lot of antibodies with same specificity as that of the membrane bound Ig. (clonal selection)  Agammaglobulinemia-inability to make Ig. Highly susceptible to infection  Function of an antibody in host defense is to recognize and bind to corresponding antigen and to target the bound antigen to other complements of the immune system...which then destroy it  One part of the antibody is high variable in that its AA sequence differs from antibody to antibody. Variable part contains site of antigen binding and confers specificity on antibody. Constant part of antibody is what interacts with other immune system components.  Immunoglobulins have 5 classes/isotypes o IgA , IgD, IgE, IgG, IgM o Distinguished on the basis of structural differences in the constant part of the molecule and have different effector function  Antibodies are glycoproteins built from basic unit of 4 polypeptide chains o 2 heavy chains (H chains) and 2 light chains (L chains) assembled into Y shape o Each arm of the Y made up of light chains paired with amino terminal part of heavy chain, covalently bonded by disulfide bond o Stem of Y consists of paired carboxy terminal portions of the 2 heavy chains (linked by disulfide bonds)  Variable region (V region)-amino terminal region of chain, vary in amino acid sequence o This variability is reason for diversity of antigen binding specificities among antibodies, because the paired V regions of heavy and light chains form antigen binding site-1 at each arm of Y (its identical) o Remaining parts of light and heavy chain are much more limited in amino acid sequence btw diff antibodies=constant region (C region)  In IgG, unstructured protion in middle of heavy chain forms flexible hinge region where molecule cleaved to produce defined antibody fragments o Fragments provide info about antibody structure and function o Fragments corresponding to arms are called Fab (fragment antigen binding) because they bind antigen o Fragment corresponding to stem is called Fc (Fragment crystallizable) because it crystallizes o The stem of a complete antibody molecule known as Fc region or Fc piece and the arms as Fab o The hinge of IgG allows the two Fabs to adopt many different orientations with respect to each other…flexibility allows antigens far away on surface of pathogens to be bound by both Fab arms  Differences in the heavy chain C regions define main isotypes or classes of Ig o IgA =  o IgD = o IgE =e o IgG =  o IgM =  Light chains have 2 isotypes= kappa () and lambda () o No functional difference found btw antibodies
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