The course of an infection can be divided into several distinct phases. & 10-2 Infectious diseases are caused by diverse living agents that replicate in their hosts

Both specific ligand and co-stimulatory signals provided by an antigen-presenting cell are required for the clonal expansion of naive T cells

Excerpt

The notion of immunity— that surviving a disease confers greater protection against it later—was known since ancient Greece. T cells are needed to control intracellular pathogens and to activate B-cell responses to most antigens HIV accumulates many mutations in the course of infection in a single individual and drug treatment is soon followed by the outgrowth of drug-resistant variants of the virus The two classes of MHCmolecule have distinct subunit structures but similar three-dimensional structures. / 3-15 Peptides are stably bound to MHC molecules, and also serve to stabilize the MHC molecule on the cell surface Many proteins involved in antigen processing and presentation are encoded by genes within the major histocompatibility complex

Proliferating T cells differentiate into armed effector T cells that do not require co-stimulation to act Memory T cells are increased in frequency and have distinct activation requirements and cell-surface proteins that distinguish them from armed effector T cells The differentiation of CD4 T cells into T H 1 or T H 2 cells determines whether humoral or cell-mediated immunity will Natural killer cells are activated by interferons and macrophage-derived cytokines to serve as an early defense against certain intracellular infections Protective immunity can be induced by injecting DNA encoding microbial antigens and human cytokines into muscleLymphocytes activated by antigen give rise to clones of antigen-specific cells that mediate adaptive immunity Some peptides and lipids generated in the endocytic pathway can be bound by MHC class I-like molecules that are encoded outside the MHC

Resolution of an infection is accompanied by the death of most of the effector cells and the generation of memory cells The variable chains of lymphocyte antigen receptors are associated with invariant accessory chains that carry out the signaling function of the receptor B-cell responses to bacterial antigens with intrinsic ability to activate B cells do not require T-cell helpThe initial interaction of T cells with antigen-presenting cells is mediated by cell-adhesion molecules T cells are specialized to recognize foreign antigens as peptide fragments bound to proteins of the major histocompatibility complex The invariant chain directs newly synthesized MHC class II molecules to acidified intracellular vesicles In the absence of inflammatory stimuli, the normal response of the mucosal immune system to foreign antigens is tolerance T cells also originate in the bone marrow, but all the important events in their development occur in the thymus