immunology allergic disorders
Cellular Components of the Immune System
Antigen Presenting Cells
Although some antigens (Ags) can stimulate the immune response directly, T cell–dependent acquired immune responses typically require antigen-presenting cells (APCs) to present antigen-derived peptides within major histocompatibility complex (MHC) molecules. Intracellular antigens(eg, viruses) can be processed and presented to CD8 cytotoxic T cells by any nucleated cell because all nucleated cells express class I MHC molecules. By encoding proteins that interfere with this process, some viruses (eg, cytomegalovirus) can evade elimination. Extracellular antigens(eg, from many bacteria) must be processed into peptides and complexed with surface class II MHC molecules on professional APCs (which specialize in presenting antigens to T cells) to be recognized by CD4 helper T (Th) cells. The following cells constitutively express class II MHC molecules and therefore act as so-called professional APCs: Dendritic cells Monocytes Macrophages B cells None Dendritic cellsare present in the skin (as Langerhans cells), lymph nodes, and tissues throughout the body. Dendritic cells in the skin act as sentinel APCs, taking up antigen, then traveling to local lymph nodes where they can activate T cells. Follicular dendritic cells are a distinct lineage, do not express class II MHC molecules, and therefore do not present antigen to Th cells. They are not phagocytic; they have receptors for the crystallizable fragment (Fc) region of immunoglobulin (Ig) G and for complement, which enable them to bind with immune complexes and present the complex to B cells in germinal centers of secondary lymphoid organs. Monocytesin the circulation are precursors to tissue macrophages. Monocytes migrate into tissues, where over about 8 hours, they develop into macrophages under the influence of macrophage colony-stimulating factor (M-CSF), secreted by various cell types (eg, endothelial cells, fibroblasts). At infection sites, activated T cells secrete cytokines (eg, interferon-gamma [IFN-gamma]) that induce production of macrophage migration inhibitory factor, preventing macrophages from leaving. Macrophagesare activated by cytokines (eg, IFN-gamma, interleukin (IL)-4, IL-13) and by various microbial components (eg, lipopolysaccharide). Activated macrophages kill intracellular organisms and secrete cytokines (eg, tumor necrosis factor-alpha [TNF-alpha], IL-10). Based on different gene expression profiles, subtypes of macrophages (eg, M1, M2), which have different roles in amplifying or limiting the inflammatory response, have been identified. B cells' primary function is to develop into plasma cells, which manufacture and secrete antibodies. 