Stryer Immunology Chapter

Stryer Immunology Chapter - 7552dc33_921-950 12:30 AM Page...

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CHAPTER 33 The Immune System We are constantly exposed to an incredible diversity of bacteria, viruses, and parasites, many of which would flourish in our cells or extracellular fluids were it not for our immune system. Remarkably, we are often even able to defend ourselves against organisms that we have never before encountered. How does the immune system protect us? The key is our ability to produce more than 10 8 distinct antibodies and more than 10 12 T-cell receptors , each of which presents a different surface for specifically binding a molecule from a for- eign organism and initiating the destruction of the invader. The presence of this remarkable repertoire of defensive molecules poses a challenge. What prevents the immune sys- tem from attacking cells that express molecules normally present in our bodies; that is, how does the immune system dis- tinguish between nonself and self ? We shall examine these questions, focusing first on the structures of the proteins par- ticipating in the molecular recognition processes and then on the mechanisms for selecting cells that express molecules use- ful for protecting us from a specific pathogen. Emphasis will be on the modular construction of the proteins of the immune system—identifying structural motifs and considering how spectacular diversity can arise from modular construction. 33.0.1 The Immune System Adapts, Using the Principles of Evolution The immune system comprises two parallel but interrelated systems. In the humoral immune response , soluble proteins 33.1 Antibodies Possess Distinct Antigen-Binding and Effector Units 33.2 The Immunoglobulin Fold Consists of a Beta-Sandwich Framework with Hypervariable Loops 33.3 Antibodies Bind Specific Molecules Through Their Hypervariable Loops 33.4 Diversity Is Generated by Gene Rearrangements 33.5 Major-Histocompatibility-Complex Proteins Present Peptide Antigens on Cell Surfaces for Recognition by T-Cell Receptors 33.6 Immune Responses Against Self-Antigens Are Suppressed OUTLINE Influenza hemagglutinin Antibody Just as medieval defenders used their weapons and the castle walls to defend their city, the immune system constantly battles against foreign invaders such as viruses, bacteria, and parasites to defend the organism. Antibody molecules provide a key element in the immune system’s defensive arsenal. For example, specific antibodies can bind to molecules on the surfaces of viruses and prevent the viruses from infecting cells. Above, an antibody binds to one subunit on hemagglutinin from the surface of influenza virus. [(Left) The Granger Collection.] Dedicated to the memory of Don Wiley, a pioneer in unraveling the structural basis of immune-system function 7552dc33_921-950 9/1/05 12:30 AM Page 921
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called antibodies (immunoglobulins) function as recognition elements that bind to foreign molecules and serve as markers signaling foreign invasion (Figure 33.1). Antibodies are secreted by plasma cells , which are derived from B lymphocytes (B cells) . A foreign macromolecule that binds selectively
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Stryer Immunology Chapter - 7552dc33_921-950 12:30 AM Page...

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