As a defense against host antibodies, some bacteria have evolved the ability to make proteins that bind to the Fc region of IgG molecules with high affinity (Ka ~ 108). One such molecule, known as protein A, is found in the cell walls of some strains of Staphylococcus aureus, and another, protein G, appears in the walls of group C and G Streptococcus. By cloning the genes for protein A and protein G and generating a hybrid of both, one can make a recombinant protein, known as protein A/G, that combines some of the best features of both. These molecules are useful because they bind IgG from many different species. Thus they can be labeled with flourochromes, radioactivity, or biotin and used to detect IgG molecules in the antigen-antibody complexes formed during ELISA, RIA, or such fluorescence-based assays as flow cytometry or fluorescence microscopy. These bacterial IgG-binding proteins can also be used to make affinity columns for the isolation of IgG.
Egg whites contain a protein called avidin that binds biotin, a vitamin that is essential for fat synthesis. Avidin is believed to have evolved as a defense against marauding rodents that rob nests and eat the stolen eggs. The binding between avidin and biotin is extremely specific and of much higher affinity (Ka ~ 1015) than any known antigen-antibody reaction. A bacterial protein called streptavidin, made by streptomyces avidinii, has similarly high affinity and specificity. The extraordinary affinity and exquisite specificity of the interaction of these proteins with biotin is widely used in many immunological procedures. The primary or secondary antibody is labeled with biotin and allowed to react with the target antigen, and the unbound antibody is then washed away. Subsequently, streptavidin or avidin conjugated with an enzyme, flourochrome, or radioactive label is used to detect the bound antibody.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.