Generating Genetic Diversity: Antibodies
The restriction-modification systems discussed in earlier provide bacteria with the ability to identify and destroy foreign DNA. Multicellular organisms also must be able to identify and destroy foreign invaders. Such invaders include not only viruses but also bacte-ria and yeasts as well as multicelled parasites. Additionally, higher organisms must protect themselves against their own uncontrolled growth. From time to time, a few of their cells lose proper regulation and begin uncontrolled growth. Often these runaway cells are stopped by the immune system, but when they are not, the result is cancer.
Similar to a bacterium’s “immunity” to infection by phage lambda, an organism can be resistant to infection by a parasite through the action of a specific protein-mediated interaction. The specific portion of the immune response of vertebrates can be considered to consist of two steps: first, recognizing a macromolecule that is foreign to the organism; and second, doing something with the recognized macro-molecule. Because of the complexity of the immune system, this chapter will be concerned primarily with only a portion of the first part of the system, that involving adaptive and specific recognition of foreign molecules. The core of this response is a set of rearrangements of DNA segments and alterations in DNA sequences that generate a multitude of genes, each slightly different from the others. These code for the many different proteins that recognize the foreign molecules.
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