Abbas chapter 5 Flashcards
What 3 classes of molecules are used by the adaptive immune system to bind antigens?
Antibodies, MCH molecules & T-cell antigen receptors.
What 2 forms/states do antibodies reside in?
Membrane bound on B lymphocytes (antigen receptor) or secreted antibodies that reside in the circulation, tissues, and mucosal sites neutralize toxins, prevent the entry and spread of pathogens, and eliminate microbes.
Describe the antibody-mediated effector functions?
1)Neutralization of microbes or toxic microbial products.
2)Activation of the complement system.
3)Opsonization of pathogens for enhanced phagocytosis.
4)Antibody-dependent cell-mediated cytotoxicity; antibodies target infected cells for lysis by cells of the innate immune system.
5) Antibody-mediated mast cell activation to expel parasitic worms.
What is anti-serum?
Any serum sample that contains detectable antibody molecules that bind to a particular antigen.
What does Immunoglobulin (Ig) refer to?
Immunoglobulin (Ig), refers to the immunity- conferring portion of the gamma globulin fraction.
Describe the structure of an antibody.
An antibody molecule has a symmetric core structure composed of two identical light chains and two identical heavy chains. Antibodies are flexible which allows them to bind to different arrays of antigens.
What defines an antigen, and what types of molecules can act as antigens?
An antigen is any substance that may be specifically bound by an antibody molecule or T-cell receptor (irrespective of whether they stimulate an immune response).
E.g Act as antigens:
1) To antibodies: almost every kind of biologic molecule including simple intermediary metabolites, sugars, lipids, autocoids, hormones, macromolecules such as complex carbs, phospholipids, nucleic acids & proteins 2) To T cells: peptides.
Describe the general structure of an antibody. What are the roles of its variable & constant regions?
An antibody molecule has a symmetric core structure composed of 2 identical light chains + 2 identical heavy chains. Both light & heavy chains contain a series of repeating homologous units (each about 110 amino acid residues in length which fold independently to form Ig domain).
Both heavy & light chains consist of:
1) An amino-acid terminal variable (V) region that participate in antigen recognition & distinguish between antibodies made by different clones of B cells.
2) A carboxyl-terminal constant (C) region. The C region in heavy chains mediate effector functions. Do not participate in antigen recognition or are directly attached to cell membranes.
Variable:
Most of the sequence differences and variability among
different antibodies are confined to three short stretches in the V region of the heavy chain & 3 stretches in V region of the light chain called hypervariable segments (corresponding to the 3 protruding loops connecting adjacent strands of beta sheets) also called complementarity-determining regions (CDR). 3 hypervariable regions of Vl + Vh = antigen binding surface. Most extensive contact is with the 3rd hypervariable region; also the most variable of the CDRs.
Constant:
Antibody molecules can be divided into distinct classes (isotopes) and subclasses on the basis of differences in the structure of their heavy chain C regions. Different isotypes and subtypes of antibodies perform
different effector functions. Isotypes & subtypes differ in their C regions & therefore what they bind to & what effector function they want to perform.
Secreted and membrane-associated antibodies differ in
the amino acid sequence of the carboxyl-terminal end of the heavy chain C region.
The heavy chain C regions of all antibody molecules of one isotype or subtype have essentially the same amino acid sequence. Heavy chains are designated by the letter of the Greek alphabet corresponding to the isotype of the antibody: IgA1 contains α1 heavy chains; IgA2, α2; IgD, δ; IgE, ε; IgG1, γ1; and IgM, µ. C regions in human IgM & IgE contain 4 tandem Ig domain; IgG, IgA & IgD only contain 3. Different isotypes & subtypes of antibodies perform different effector functions
How do antibodies recognize antigens? What makes this interaction highly specific?
Precise fit
Explain the difference between membrane-bound antibodies and secreted antibodies.
In the secreted form (blood & extracellular fluid) the carboxyl-terminal portion is hydrophilic. The membrane-bound form contains a carboxyl-terminal stretch that includes a hydrophobic α-helical transmembrane anchor region followed by an intracellular juxtamembrane positively charged stretch which helps anchor the protein in the membrane.
What is meant by the ‘half-life’ of antibodies, & why is it important?
How can the diversity of the antibody repertoire be explained in terms of genetic organization & expression?
Discuss the importance of monoclonal antibodies & their applications.
How do structural features of antibodies relate to their function in the immune response?
Antibodies are flexible which allows them to bind to different arrays of antigens. Flexibility conferred largely by hinge region between Ch1 & Ch2.
An antibody molecule has a symmetric core structure
composed of two identical light chains and two identical
heavy chains. Both heavy & light chains consist of:
1) An amino-acid terminal variable (V) region that participate in antigen recognition & distinguish between antibodies made by different clones of B cells.
2) A carboxyl-terminal constant (C) region. The C region in heavy chains mediate effector functions. Do not participate in antigen recognition or are directly attached to cell membranes.
Because the core has 2 of each light & heavy chains every antibody has at least 2 antigen binding sites.
Heavy chains= 1 Ig domain in V region + 3-4 Ig domains in C region.
Light chains = 1 Ig domain in both C & V region
If fragmented; 3 pieces; 2 pieces identical = Vl-Cl (complete light chain) associated with Vh-Ch1fragment of heavy chain–> retain antigen binding capacity (fragment, antigen binding). 3rd piece - heavy chain domains Ch2 & Ch3 which have the propensity to self associate & crystallise (fragment crystallisation; fc)
What mechanisms are involved in the binding of antibodies to antigens?
Explain the concept of antigenic determinants or epitopes. How do antibodies recognize
these structures?
Where are the Ig synthesized ? Describe the process.
Circulating IgE has a half-life of 2 days (although cell-bound IgE associated with the high-affinity IgE receptor on mast cells has a very long half-life).
Circulating IgA has a half-life of 3 days (although most IgA is produced at mucosal sites & is secreted
directly in the lumen of the gut or airway).
Circulating IgM has a half-life of 4 days.
Circulating IgG has a half-life of 21-28 days.
Why, & how is that use as a therapeutic advantage?
Antibodies have a high degree of specificity for antigens. However, we can sometimes see what is called ‘cross-reaction’. Define this feature.
An individual is capable of producing millions of structurally distinct antibodies, each with
a specificity. The ability of antibodies to specifically bind a large number of different antigens is a reflection of antibody diversity, and the total collection of antibodies with different specificities represents the antibody repertoire.
What is true and false ?
Correct the false sentences:
1) Only the B lymphocytes present the genetic mechanisms that generate such a diverse repertoire.
2) The ability of antibodies to specifically recognize a wide variety of antigens with varying affinities reflects the properties of the C regions.
3) About millions of inherited germline DNA sequences are involved to form functional genes that encode de V regions of heavy & light chains.
4) The recombination process is influenced by the environment in which an individual lives.
5) During the recombination process, some nucleotide sequences are randomly added & that participate to increase the antigen specificity.
1) True.
2) False.
3) False.
4) False; its random.
5) True (?CHECK)
Explain what is the affinity maturation of the antibody-antigen bindings.
Complete theses sentences :
‘Many of the effector functions of antibodies are mediated by ……….. portions, and Ig
isotopes that differ in these Fc regions perform distinct functions. For example, IgG coats
microbes and target them for ……………………. [explain how]. In contrast, IgE binds to
……………………………………. and triggers their degranulation. Another Fc-dependent
effector mechanism of humoral immunity is activation of the …………………….. pathway of
the complement system, which generates ………………………… mediators and promotes
microbial phagocytosis and ……………………….. . This is initiated by a protein called
……………….. which binds to the Fc portions of Ag-complexed ………… or ……….. .
Why can’t the free Ig initiate effector functions ?
What is the isotope (or class) switching ? Is that a good thing or a bad thing, & why?
Considering the spatial arrangement of different epitopes on a single protein, how would you define ‘nonoverlapping’ & ‘overlapping’ determinants, & the ‘allosteric effects’ of some determinants ?
What are ‘conformational epitopes’, ‘linear epitopes’ and ‘neoantigenic epitopes’?
About antigens, what sentences are true or false? For false sentences, correct them.
1) All kind of biological molecules may be bound by an antibody molecule (simple
metabolites, sugars, hormones, nucleic acids,…).
2) All antigens are immunogens.
3) The portion of an Ig that binds to a macromolecule is the epitope & the portion of this macromolecule that interacts with the Ig is the paratope.
4) A synonym for ‘small chemicals’ is ‘hapten’ & the large molecule to which it is conjugated is called ‘carrier’.
5) The ‘polyvalency’ or ‘multivalency’ is the capacity of a macromolecule to activate different lymphocytes at the same time & thus result in many clones.
1) True.
2) False.
3) False.
4) True.
5) False.
Complete the following text :
The recognition of antigens by antibodies involves ……………… binding of various types (including ……………………… forces, hydrogen bonds, van des Waals forces and…………………. interactions). The strength of the binding of an antibody and an epitope of
an antigen is called the …………………. of the antibody and is represented by a………………….. constant (Kd) which indicates how easy it is to ………………. an antigen-antibody complex into its constituents by changing their ………………… . Because the ……………………………… of antibodies gives them flexibility, a single antibody may attach to a single multivalent antigen by more than one binding site. For IgE and Ig……….,this attachment can involve, at most, 2 binding sites. For pentameric ……….., a single antibody may bind at up to 10 different sites. The overall strength of multiple attachment between the …………………….. antigen and the antibody is called the ……………………… and is much greater than the affinity of one antigen-binding site.
What is an Ig domain composed of?
Contains 2 layers of beta pleated sheet, each layer composed of 3-5 strands of antiparallel polypeptide chain. The 2 layers are held together by a disulphide bridge & adjacent strands of each beta sheet are connected by short loops (the amino acids in some of these loops are the most variable & critical for antigen recognition).
