principles underlying such hierarchically arranged cooperative processes involving feedback is still rudimentary. With an understanding of these principles, scientists will be able to create materials with unprecedented functional capabilities.

The process of unraveling the principles will be expedited by bringing theoretical and computational approaches together with experimental approaches such as genetic, biochemical, and imaging experiments. Expertise in the statistical mechanics of complex systems from the study of soft materials will also be a great asset. In addition, the effects of stochastic fluctuations must be included and properly treated. Ideas from network theory may also help researchers to understand cooperative processes that take place on very different length and time scales. Research at the crossroads of statistical mechanics, materials science and engineering, and molecular and cell biology should pay dividends for both fundamental biological research and the understanding of these essential highly cooperative processes and interactions. Some examples of research focused on understanding the specificity of detection and the precision of response in biological systems follow in the next subsections.

Cells

Many living cells carry out specific functions when they sense certain external stimuli. One example of cells that function this way is the T lymphocytes, also known as T cells. They have evolved to deal with pathogens that are no longer in the blood or on mucosal surfaces but have penetrated other cells. Because they combat pathogens that have not been previously encountered, they are critical components of the adaptive immune system in higher organisms such as vertebrate animals. They can rapidly and sensitively detect the presence of biological and chemical hazards. Moreover, they can detect minute quantities of hazardous molecules without frequent false positive responses. Thus mimicking T cells would be one way of overcoming an important challenge facing society today—namely, the rapid and sensitive detection of biological and chemical hazards in the environment, including unknown pathogens that could be engineered, perhaps from existing agents.

Specialized cells, called antigen-presenting cells (APCs), display molecular signatures of the pathogen on their surface (Figure 2.1). Antigen-derived proteins are cut up into small peptide fragments by enzymes in APCs. These peptide fragments can then bind to major histocompatibility proteins, and this complex of peptide (p) and major histocompatibility (MHC) is the molecular flag of the pathogen displayed on the surface of APCs. Peptides derived from proteins of the host organism can also bind to MHC molecules, and these self pMHC molecules are also displayed on APC surfaces. T cells can detect as few as 10 antigen-derived pMHC molecules in a sea of tens of thousands of self pMHC molecules.



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