1. Definition of Antigen-Antibody Reactions2. Antibodies3. Antigens4. Nature of Antigen- Antibody reactions5. Characteristics of antigen-antibody reactions6. Types of antigen-antibody reactions7. Consequences of antigen-antibody reactions
It is the reaction between an antigen and an antibody when they come in contact. This occurs when antigens enter the body. When antigens enter the body, signals are sent out and in response to that antibodies are made specifically to destroy the antigens. It is part of the Adaptive(Specific) immune system.
Antibodies are also known as Immunoglobulin (Ig) These are globulin proteins that are found in the blood or other bodily fluids of vertebrates. Antibodies are used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. Antibodies are produced by the Plasma Cells and are the receptors of the B- cells. An Antibody is typically a Y-shaped structure consisting of four polypeptide chains- two Heavy chains and two Light chains. Each arm of the „Y‟ bears an antigen binding site.
An antigen is any substance that triggers your immune system to produce antibodies against it. It may be a foreign substance from the environment such as chemicals, bacteria, viruses, or pollen. It may also be formed within the body, as with bacterial toxins or tissue cells. Antigens have different structures and types.
Antigen is a substance that reacts with the products of a specific immune response( specific antibodies) Immunogen is a substance that induces a specific immune response
1. Lock and key concept - Fab ( antigen- binding fragment) is the combining site which is constructed by Hypervariable regions( H1-H4) of the heavy and light chains that interacts with an antigenic determinant(epitope) which forms a lock and key concept where lock= antibody and key= antigen - X-ray crystallographers studies of Ag-Ab interactions shows that antigenic determinant is nestled in a cleft formed by the combining sites of the antibody as illustrated…
2. Non- Covalent Bonds There are Four invisible forces that enables the binding of the antigen and the antibody and these are: a. Hydrogen Bond - When the antigen and antibody are in very close proximity, relatively weak hydrogen bonds can be formed between hydrophilic groups (e.g., OH and C=O, NH and C=O, and NH and OH groups). b. Electrostatic Bond - Electrostatic bonds result from the attraction between oppositely charged ionic groups of two protein side chains, for example, an ionized amino group (NH4) on a lysine in the antibody and an ionized carboxyl group(COO) on an aspartate residue in the antigen.
c. Hydrophobic Bond - the attractive force between molecules due to the close positioning of non- hydrophilic portions of the two molecules ( ie antigen and antibody)d. Van der Waal’s forces - These forces depend upon interactions between the “electron clouds” that surround the antigen and antibody molecules. The interaction has been compared to that which might exist between alternating dipoles in two molecules, alternating in such a way that at any given moment oppositely oriented dipoles will be present in closely apposed areas of the antigen and antibody molecules.3. Reversibility - it is reversible since the Ag- Ab rxns are via non- covalent bonds.
ANTIBODY AFFINITY Strength or force of attraction of a rxn btwn a single antigenic determinant (epitope) at a single combining site on the Ab. ↑ the affinity of Ab for Ag, the more stable the interaction will be.
ANTIBODY AVIDITY Overall strength of binding of several antibodies produced in response to an immunogen, which presents several different epitopes to the immune system. ↑ the avidity, the more stable the Ag-Ab reaction
Fig. 8.3 Diagrammatic representation of theavidity concept. The binding of antigenmolecules by several antibodies of differentspecificities (A) stabilizes the immunecomplex, since it is highly unlikely that allAgAb reactions dissociate simultaneously atany given point of time (B). (Redrawn fromRoitt, I. Essential Immunology, 4th ed.Blackwell, Oxford, 1980.)
ANTIBODY SPECIFICITY refers to the ability of an individual antibody combining site to react with only one epitope or the ability of a population of Ab molecules to react with only one antigen. In general, there is a high degree of specificity in Ag-Ab rxns.
CROSS REACTIVITY refers to the ability of an individual antibody combining site to react with more than one epitopes. It arises because the cross reacting antigen shares an epitope in common with the immunizing antigen or becoz it has an epitope which is structurally similar to one of the immunizing antigen.
Fig. 8.4 Diagrammatic representationthe concept of cross-reaction betwecomplex antigens.An antiserum containing severalantibody populations to thedeterminants of a given antigen willreact with other antigens sharingcommon or closely relateddeterminants. The avidity of thereaction will decrease with decreasistructural closeness, until it will nolonger be detectable. The reactivityof the same antiserum with severalrelated antigens is designated as croreaction. (Redrawn from Roitt, I.Essential Immunology, 4th ed.Blackwell, Oxford, 1980.)
AGGLUTINATION REACTIONS- the aggregation by antibody of already insoluble particles or cells, e.g., bacteria or red cells, into larger clumps.
Fig. 8.8 IgM antibodies are moreefficient in inducing red cellagglutination. Red cells remain atthe same distance from eachother due to their identicalelectrical charge (zeta potential).IgG antibodies are not largeenough to bridge the spacebetween two red cells, but IgMantibodies, due to theirpolymeric nature and size, caninduce red blood cellagglutination with considerableease.
PRECIPITATION REACTIONS A. Quantitative Reaction: - adding increasing amount of antigen to a known amount of antibodies results in a progressive increase in the amount of specific precipitate, which reaches a maximum level and then declines. - precipitation occurs when there is more binding and cross linking take place and it‟s at its highest peak when there are no free- antibodies and free- antigens in the solution.
B. Qualitative Reactions - various procedures are used to determine the presence, identity, and specificity of soluble antigens and antibodies. - Procedures include: - Ring precipitin tests, agar diffusion tests, and immunoelectrophoresis are commonly done in the laboratory.
Fig. 8.7 Diagrammatic representation ofa reaction of double immunodiffusion.Antigen and antibody are placed inopposite wells carved in a semi-solidmedium (e.g., agarose gel). Bothantigen and antibody diffuse in alldirections and towards each other,reacting and eventually reachingequivalence, at which point a linearprecipitate appears between theantigen and antibody wells.
Opsonization Antigen- Antibody rnx ↓ Antibodies unfold ↓ interаcts with Fc receptor on phаgocytic cells ↓ phаgocytosis
Fc – receptor mediated cell activation ag-ab complexes + phagocytic cells through their Fc receptors ↓ activates ingesting cell (phagocytes)→ inflammatory reaction (less favorable outcome) ↓ enhancement of its microbicidal activity. NB: Another adverse reaction results from the engagement of Fc receptor–bound IgE on basophils and mast cells with their corresponding antigen. The result of this reaction is the release of the potent mediators that trigger an allergic reaction .
Complement Activation Complement activation is an extremely important amplification mechanism, which mediates antibody-dependent neutralization and elimination of infectious agents. There аre two mechаnisms involved:1. Generation of C3b →deposited on the membrane of a microorganism →facilitates phagocytosis by cells with C3b receptors.
2. Disruption of lipid bilayers that depends on the generation of the late complement components (C6–C9). late complement components (C6–C9) → properly assembled on a cell membrane → induce formation of transmembrane channels → cell lysis.
Neutrаlisаtion аntibodies bind to bаcteriа, viruses, or toxin (Ag- Ab rnx) → prevents the interaction of the microbial agents or their products with the receptors that mediate their infectiveness or toxic effects → infectious agent or the toxin become harmless( neutralized.)
Antigen- Antibody reactions are interactions between antigens and antibodies when they come into contact. Nature of the Ag-Ab reactions are as follows: Lock and Key Concept; Non- covalent bonds (Hydrogen, Electrostatic, and Hydrophobic bonds, and Van der Waal‟s Forces; and Reversibility. Characteristics of Ag- Ab reactions are: Affinity; Avidity; Specificity; and Cross- reactivity. The Specific Types of Ag- Ab reactions include: Agglutination and Precipitation Reactions Consequences of the Ag-Ab reactions are: Opsonization; Fc- receptor mediator Cell activation; Complement Activation; and Neutralization.
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Figure 1 In an antigen, the same antigenic determinant repeated many times Figure 3Figure 2 Hapten-carrier conjugates have nativeT-dependent antigens are antigenic determinants of the carrier ascharacterized by a few copies of many well as new determinants of thedifferent antigenic determinants hapten
Figure 4Antigenic determinants are usually Figure 5limited to those portions of the antigen Superantigens activate a largethat are accessible to antibodies fraction of T cells in contrast toshown in black for this iron-containing conventional T-dependentprotein antigens