Exam

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Antigens

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Antigens are foreign substances that provoke a response from the immune system. Antigens can come from pathogenic bacteria, viruses, fungi or protozoa or they can be substances such as pollen, insect venom and transplanted tissue.

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Antibodies

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Antibodies are proteins produced by the immune system in response to the presence of antigens. Antibodies bind to antigens with a high degree of specificity.

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Active immunityis

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Active immunityis acquired when an individual develops their own antibodies in response to exposure to an antigen.

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Humoralor antibody-mediated immunity

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•Humoralor antibody-mediated immunityis gained through the production of antibodies against antigens. B lymphocytes (B cells)are specialized to produce antibodies.

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Cell-mediated immunity

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Cell-mediated immunityrefers to the T lymphocytes (T cells)that attack foreign substances and cells. Both antibodies and T cells are needed to mount an immune response. The body will not produce enough antibodies without the T lymphocytes.

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binding of antigen and antibody

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*Antigens are usually proteins or large polypeptides found on capsules, cell walls, flagella, fimbriae, toxins, pollen, egg white, blood cells, transplanted tissue, etc.
•The particular region of an antigen that specifically interacts with an antibody is known as an epitope.
•The shape and chemical properties of the antibody and the epitope are complementary to each other to enable binding.

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haptens

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•Haptens are antigens with a molecular weight that is so small (less than 10,000 atomic mass units) that the molecule does not provoke an immune response. Penicillin and the toxin in poison ivy are too small to initiate an immune response independently.
•An immune response requires some kind of a carrier molecule. A covalent bond forms between the hapten and a carrier protein such as the albumin protein suspended in the blood.
•Antibodies develop against the hapten-carrier complex. These antibodies will later be able to recognize haptens in the absence of the carrier molecule.

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variable and constand regions

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•Each antibody consists of two identical heavy chainsand two identical light chains. These chains are held together by disulfide bonds between cysteine amino acids.
•Variable regionsare found at each end of the heavy and light chains. The amino acid sequences and tertiary structures of these variable regions vary between antibodies.
•The heavy and light chains fold to form pockets called antigen-binding sites. The antigen-binding sites will have the shapes and chemical properties suitable for binding to antigens.
--•Each Y-shaped antibody monomer has two identical antigen-binding sites.
•The remainder of the antibody consists of the constant region.
•The stem of the antibodies is known as the FCregion. Once the antigen-binding regions react with antigens, the FCregion may bind to complement proteins.
•Alternatively, the FCregion may bind to a cell before the antibody reacts with antigens.

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igG

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•Ig stands for immunoglobulin.
•The IgG antibodies make up 80% of all antibodies found in serum. These monomers are small enough to pass through a mother’s placenta and protect her fetus.

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igM

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•IgMantibodies are pentamers of antibodies held together by joining polypeptides. These antibodies often appear early in infection and are useful for aggregating the antigens and for activation of the complement system

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igA

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•IgAantibodies are prevalent in mucus, saliva, tears and milk.
•IgA antibody monomers prevent pathogens from attaching to surfaces coated by mucous membranes.
•A joining chain can be used to link two monomers to form a dimer.
•A secretory chain is a short polypeptide that also attaches to the two dimers.
•The secretory chain prevents the digestion of the IgA antibodies when they are delivered to infants through milk.

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igD

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•IgDantibodies make up 0.2% of all antibodies. The IgD antibodies are attached to B cells that have not been exposed to antigens and may have a regulatory function.

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igE

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•IgEantibodies make up 0.002% of all antibodies. The FCstem regions of IgE antibodies are attached to mast cells or basophils that are involved in the allergic response. Binding of an allergen to IgE antibodies causes the release of histamines by mast cells and basophils.

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Antibodies fight pathogens in several ways

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•Each antibody has at least two antigen-binding sites. Therefore, large antibody-antigen complexes can form.
•Agglutinationis the forming of large clumps of antibody and antigens. Because pathogens are trapped in these clumps, they cannot infect cells. Agglutination makes phagocytosiseasier.
•Other antibodies act as opsonins. Opsoninshelp phagocyticcells attach to pathogens, particularly those pathogens with capsules.
•Antibodies can also activate the complement system. Complement proteins can lyseopen bacteria.
•The activation of complement by antibodies also helps to activate the immune response.
•Antibodies can also neutralize pathogenic bacteria and viruses by physically blocking these pathogens from adhering to mucus membranes or attaching to target cells. Antitoxins can also neutralize the exotoxins released by pathogens.
•Antibodies can also activate nonspecific natural killer cells. These natural killer cells can be used to combat larger pathogens such as flatworms.