macrophages express a variety of receptors, each of which is able to recognize specific components of microbes. Some, like the mannose and glucan receptors and the scavenger receptor, bind cell-wall carbohydrates of bacteria, yeast, and fungi. the toll-like receptors (tlrs) are an important family of pattern recognition receptors present on macrophages, dendritic cells, and other immune cells. tlrs recognize different microbial components; for example, a heterodimer of tlr-1 and tlr-2 binds certain lipopeptides from pathogens such as gram-positive bacteria, while tlr-4 binds both lipopolysaccharides from gram-negative and lipoteichoic acids from gram-positive bacteria.
Fig. 1.10 Infection triggers an inflammatory response. macrophages encountering bacteria or other types of microorganisms in tissues are triggered to release cytokines (left panel) that increase the permeability of blood vessels, allowing fluid and proteins to pass into the tissues (center panel). macrophages also produce chemokines, which direct the migration of neutrophils to the site of infection. the stickiness of the endothelial cells of the blood vessel wall is also changed, so that circulating cells of the immune system adhere to the wall and are able to crawl through it; first neutrophils and then monocytes are shown entering the tissue from a blood vessel (right panel). the accumulation of fluid and cells at the site of infection causes the redness, swelling, heat, and pain known collectively as inflammation. neutrophils and macrophages are the principal inflammatory cells. later in an immune response, activated lymphocytes can also contribute to inflammation.
Vasodilation and increased vascular permeability cause redness, heat, and swelling fuids protein neutrophil Infammatory cells migrate into tissue, releasing infammatory mediators that cause pain monocyte
Instead of presenting all the cytokines together all at once, we introduce each cytokine as it arises during our description of cellular and functional responses. We list the cytokines, their producer and target cells, and their general functions in Appendix III.
Chemokines are a specialized subgroup of secreted proteins that act as chemoattractants, attracting cells bearing chemokine receptors, such as neutrophils and monocytes, out of the bloodstream and into infected tissue (Fig. 1.10). Beyond this role, chemokines also help organize the various cells in lymphoid tissues into discrete regions where specialized responses can take place. There are on the order of 50 different chemokines, which are all related structurally but fall into two major classes. Appendix IV lists the chemokines, their target cells, and their general functions. We will discuss chemokines as the need arises during our descriptions of particular cellular immune processes.
The cytokines and chemokines released by activated macrophages act to recruit cells from the blood into infected tissues, a process, known as inflammation, that helps to destroy the pathogen. Inflammation increases the flow of lymph, which carries microbes or cells bearing their antigens from the infected tissue to nearby lymphoid tissues, where the adaptive immune response is initiated. Once adaptive immunity has been generated, inflammation also recruits these effector components to the site of infection.