Antibodies (Immunoglobulins) Chapter 4

Antibody structure and function Antibodies belong to a family of globular proteins called immunoglobulins. These proteins can react and bind to antigen. Antibodies are secreted and they also exist as the B-cell receptor (BCR) on the surface of the B-cell.

Diversity Antibodies come in millions of different amino acid sequences and are the most diverse proteins known. Because the amino acid sequence differs in the arms of various antibody molecules, each different antibody can bind specifically to one unique epitope. Thus, the arms of an antibody molecule confer the specificity of responses that a host can mount against antigens. The stem region of an antibody molecule bear its biological activity and defines whether the response against a particular antigen will lead to complement- mediated lysis, enhanced phagocytosis, or (in some cases) allergy. These activities start once antibodies bind to antigen.

Biological Activities The ability to trigger protective physiological activities usually after binding to antigen. This would include: -Opsonization, -activation of complement, -allergic responses, -neutralize toxins, -neutralize viruses and other activities

Antibody fragments -Digestion With Papain Yields -Pepsin Digestion 2 identical Fab and 1 Fc Fab Because Fragment That is Antigen Binding Fc Because Found To Crystallize In Cold Storage -Pepsin Digestion F(ab`)2 No Fc Recovery, Digested Entirely -Mercaptoethanol Reduction (Eliminates Disulfide Bonds) And Alkylation Showed

Antibodies classes The class of antibody is also called the isotype of the antibody. There are 5 isotypes or classes of antibodies in humans: IgM, IgG, IgA, IgE, IgD; these classes are defined by the class of Heavy Chain or H-chain: IgM - m chain -mu IgG -g chain -gamma IgA -a chain- alpha IgE -e chain -epsilon IgD - d chain- delta Differ in structure and in biological activity.

IgG Structure

IgG Functions Most abundant of all Ig classes Longest half-life of all the Ig classes Agglutinates particulate antigens Precipitates soluble antigens This is the only antibody class that crosses the placenta Opsonizes antigen. IgG that is bound to cells or IgG that is aggregated in antigen-antibody complexes can activate complement. IgG is good at neutralizing toxin IgG can interfere with bacterial virulence features such as motility and adherence to tissues IgG can block virus ability to adhere to target cells IgG is the main antibody found in the secondary immune response

IgG functions continue Most abundant immunoglobin 80% of serum Ig ~10mg/mL IgG1,2,3,4 (decreasing serum concentration) IgG1, IgG3 and IgG4 cross placenta IgG3 Most effective complement activator IgG1 and IgG3 High affinity for FcR on phagocytic cells, good for opsonization

IgM structure

IgM Function IgM monomer is the B-cell receptor of the mature B-cell IgM is the first antibody made in an immune response, about 7 to 10 days after initial exposure IgM is the first antibody made in life, about 5 months in utero IgM is the most primitive antibody class, found even in primitive vertebrates. IgM is the antibody made to T-independent antigens, polysaccharides IgM is a very good agglutinating antibody Natural isohemagglutinins are IgM class IgM attached to antigen is a very good activator of complement IgM does not cross the placenta 5-10% of serum immunoglobulin 1.5mg/mL

IgA Functions 10-15% of serum IgG 5-15 g of IgA released in secretions!!!! IgA is the secretory antibody, found in all secretions including colostrum and milk saliva tears mucus. IgA is particularly important for protection against respiratory and gastrointestinal infectious agents. IgA is important for passively acquired immunity of nursing baby. IgA makes lysozyme work better, especially against gram-negative bacteria IgA neutralizes virus IgA is transported across membranes via the poly-Ig receptor

Draw the structure of IgA

IgE functions Very low serum concentration, 0.3g/mL IgE is made in response to parasitic worms. When bound to the parasite, it triggers a very strong inflammatory response to the worms. IgE is the antibody that triggers allergy IgE binds to Fc receptors on the surface of mast cells and basophils Binding causes degranulation (Histamine Release)

Draw the structure of IgE?

Cross-Linkage of Bound IgE Antibody With Allergen Causes

IgM and IgD, Expressed on B-cell Surface We Do Not Know Any Other Biological Effector Activity Low serum concentrations, ~30g/mL

Monoclonal Antibody Applications Diagnostic Tests Abs are capable to detect tiny amouns (pg/mL) of molecules Ex. Pregnancy hormones Diagnostic Imaging mAbs that recognize tumor antigens are radiolabeled with iodine I-131 Immunotoxins mAbs conjugated with toxins mAbs To Clear Pathogens www.elusys.com