T cells do not interact with free antigens or with antigenic sites in the surfaces of foreign microorganisms. Instead, T cells respond only to cells bearing both a self MHC antigen and an antigenic determinant from a foreign source (i.e., from bacteria, viruses, etc.). Thus, two stimuli are needed to trigger the proliferation and terminal differentiation of the required T-cell [...]
Structure of the Lattice: Conventional transmission electron microscopes have accelerating potentials of several thousand volts and produce an electron beam that penetrates tissue and cell slices (i.e., sections) having thicknesses of about 0.2 nm. The development in the 1970s of high-voltage electron microscopes capable of accelerating electrons over a potential of one million volts has made it possible to study [...]
We will be concerned with the organization, composition, and assembly of the cytoplasmic ribosomes of prokaryotic and eukaryotic cells. The ribosomes present in cytoplasmic organelles (e.g., chloroplast and mitochondrial ribosomes). Although functionally analogous, many differences exist between the ribosomes of prokaryotic and eukaryotic cells (Table 22-2). Considerably more is known about the structure and composition of bacterial ribosomes than ribosomes [...]