المشاركة الأصلية كتبت بواسطة Gardi
Antigens
Substances that can be recognized by the immunoglobulin receptors of B cells , or by the T- cell receptor when complexed with MHC, are called antigens. The molecular properties of antigens and the way in which these properties ultimately contribute to immune activation are central to our understanding of the immune system.
Antigens are molecules that react with antibodies' whereas immunogens are molecules that induce an immune response. In most cases' antigens are lmmunogens, and the terms are used interchangeably' However' there are important exceptions' eg' haptens . A hapten is a molecule that is not immunogenic by itself but can react with specific antibody . Hapten are usually small molecules , but some high – molecular – weight nucleic acids are hapten as well. Many drugs, eg, penicillin, are haptens , and the catechol in the plant oil that causes poison oak and poison ivy is a hapten.
Hapten is not immunogenic because they cannot activate helper T cells .The failure of hapten to activate is due to their inability to bind to MHC proteins ; they cannot bind because they are not polypeptides and only polypeptides can be presented by MHC protein .Furthermore , haptens are univalent and therefore cannot activate B cells by themselves . Although hapten cannot stimulate a primary or secondary response by themselves , they can do so when covalently bound to a “ carrier ” protein ( Figure 1) .In this process , the hapten interacts with an IgM receptor on the B cell and the hapten-carrier protein complex is internalized .A peptide of the carrier protein is presented in association with class II MHC protein to the helper T cells . The activated helper cells then produces interleukins , which stimulate the B cells to produce antibody to the hapten .
Two additional ideas are needed to understand how hapten interact with our immune system .
The First is that many hapten , such as drugs (eg, penicillin ) and poison oak oil , bind to our normal proteins , to which we are tolerant . The hapten –protein combination now becomes immunogenic ; ie, the hapten modifies the protein sufficiently such that when the hapten- peptide combination is presented by the MHC protein , it is recognized as foreign.
The second idea is that although most hapten are univalent , type I hypersensitivity reactions such as anaphylaxis require cross-linking of adjacent IgEs to trigger the release of the mediators , but when many hapten molecules are bound to the carrier protein , they are arranged in such away that cross-linking can occur . An excellent example of this is penicilloyl polylysine , which is used in skin test to determine whether a patient is allergic to penicillin . Each lysine polylysine has a penicillin molecule attached to it . These univalent penicillin molecules from a “multivalent ” array and can cross-linking adjacent IgEs on the surface of mast cells . The consequent release of mediators causes a “ wheal and flare ” reaction in the skin of the penicillin –allergic patient .
The interaction of antigen and antibody is highly specific , and this characteristic is frequently used in the diagnostic laboratory to identify microorganisms . Antigen and antibody bind by weak forces such as hydrogen bonds and Van der Waals forces rather than by covalent bonds . The strength of the binding ( the affinity ) is proportionate to the fit of the antigen with its antibody-combining site , ie, its ability to form more of these bond . The affinity of antibodies in creases with successive exposure to the specific antigen . Another term , avidity , is also used expess certain aspcts of binding . It need not concern us here .هاي تشرحلك عن المستضدات و المستضد وكيف الأجسام المضادة ترتبط مع بعضها مثلا لمن يكون المريض عنده حساسية من البنسلين
