Reticuloendothelial system (RES) is one of the two overlapping compartments of the immune system. Cells of the RES provide natural immunity against microorganisms by 1) a coupled process of phagocytosis and intracellular killing, 2) recruiting other inflammatory cells through the production of cytokines, and 3) presenting peptide antigens to lymphocytes for the production of antigen-specific immunity. The RES consists harbors macrophages, which are cells that play major roles in 1) defending against many microbial pathogens and 2) generating specific immune responses by processing protein antigens and presenting the resultant peptide antigen fragments in the context of MHC molecules to T cells. The system consists of 1) monocytes in the blood, 2) macrophages in the liver, spleen, lymph nodes, thymus, bone marrow, connective tissues, and submucosal tissues of the respiratory and alimentary tracts, 3) dendritic cells in lymph nodes, 4) Langerhans cells in skin, and 4) glial cells in the central nervous system. Macrophages not only operate in direct defense (phagocytosis and intracellular killing) but also marshal other parts of the immune system, such as T lymphocytes.
Some functions of macrophages such as phagocytosis and intracellular killing are similar to those of neutrophils, whereas others are distinct. The distinctive features are as follows: a) They are able to reside in the RES for long periods. b) They process protein antigens and present the resultant peptide fragments to T cells in the context of Major Histocompatibility Complex class II molecules. They produce cytokines. These cells are also highly adherent, motile and phagocytic. These properties are greatest in activated macrophages, somewhat less in unstimulated macrophages, and least in monocytes. The role of these cells in processing and presenting antigens is dealt with in the next section.
A large macrophage adhering to the luminal surface of the capillary's endothelium and containing multiple organisms free within its cytoplasm.
Credit: CDC/ Dr. Ed Ewing
Monocytes and macrophages are activated by bacterial products such as endotoxin (lipopolysaccharides); autocrine agents, such as TNF-α, IL-1, and IL-8; cytokines such as interferon-γ (IFN-γ) and a special group of mediators called chemokines. Activated macrophages play a prominent effector role in cellular immunity by 1) ingesting and killing pathogens, 2) clearing immune complexes, and 3) aiding in the genesis of specific immune responses by antigen presentation.
- Medical Microbiology. 4th edition. Armond S. Goldman and Bellur S. Prabhakar