AQUIRED IMMUNITY CONT'D (LEC 4).docx

AQUIRED IMMUNITY CONT’D (818-824) • Antibody functions o Antibody binds to antigen first  Fc region of antibody-antigen complex to bind to Fc receptors on an immune cell  A single Fc receptor eliminates need for immune cells to have multiple different receptors that recognize different antigens • Immune cells are activated by any antibody-bound antigen  If immune cell is a phagocyte, Fc binding initiates phagocytosis, in which antibodies perform the following • Act as opsonins • Make antigens clump  If immune cell is a NK cell or esinophil Fc binding • Activates cytotoxic cell responses o Release chemicals that destroy antibody-bound antigen o Nonspecific response of cytotoxic cells to antibody binding is called antibody-dependent cell-mediated cytotoxicity  Antibodies enhance inflammation by • Inactivating bacterial toxins o Antibodies bind to and neutralize some toxins o When developing vaccines, it trigger antibody production without causing symptoms of disease  Activating complement  Activating mast cells • Have IgE antibodies attached to their surface • When antigens or complement proteins bind to IgE, mast cells degranulate, releasing chemicals that mediate inflammatory response PSL301H1|Lecture 4  Activate B lymphocytes • Once antigen is bound, activated B cells differentiate into plasma cells and secrete more antibodies • Some B cells differentiate into memory cells • T lymphocytes use contact-dependent signalling o Antibodies are only effective against extracellular pathogens because antibodies can only bind to soluble or exposed pathogens  Defending the body against intracellular pathogens is the role of cytotoxic T lymphocytes, which carry out cell-mediated immunity • Cytotoxic T cells bind to APC, presenting the fragments as part of their MHC o T lymphocytes develop in the thymus gland  Immature precursor cells migrate from the bone marrow to the thymus  T cell receptors are not antibodies  Can only bind to MHC-antigen complexes on the surface of an antigen- presenting cell, not free-floating antigens like B cells • Two types of MHC molecules o MHC class I  Found on all of human cells  when pathogens invade the cell, they are digested into fragments, and loaded onto the MHC-I • if a C cell encounters a host cell with foreign antigen fragments on its MHC-I, it recognizes it as defective, and kills it o MHC class II  Found primarily on APCs • Macrophages, dendritic cells, B lymphocytes  when pathogens invade the cell, they are digested into fragments, and loaded onto the MHC-II PSL301H1|Lecture 4 • if a H cell encounters an APC with foreign antigen fragments on its MHC-II, it responds by secreting cytokines to enhance the immune response • Cytotoxic T cells o T Cells destroy cells that display MHC-I antigen complexes in 2 ways  Can release perforin molecules with granzymes • Granzymes enter the target cell via perforin channels • Activate enzyme cascade, inducing apoptosis  Can instruct cells to undergo apoptosis via Fas, protein on the target membrane linked to enzyme cascade • Helper T cells o Do not directly attack pathogens and infected cells  Secrete cytokines which activate other immune cells • IFN- γ o Activates macrophages • Interleukins o Activate antibody production and T cells C o Support actions of mast cells and eosinophils • CSF o Enhance WBC production  Bind to B cells and promote differentiation into plasma cells and memory B cells • Immune response pathways o Bacterial invasion causes inflammation  Entry of bacteria sets off several interrelated reactions • Activity of complement system PSL301H1|Lecture 4 o Components of bacterial cell wall activate complement system  Chemotaxins  Opsonins o Causes degranulation of mast cells and basophils  Cytokines secreted by mast cells act as chemotaxins  Histamine dilates blood vessels and increases capillary permeability • Increased blood flow causes redness and warmth  Plasma protein escaping into interstitial space pull water with them, causing swelling o Ends with formation of MAC molecules that insert themselves into the bacterial wall, causing them to lyse • Activity of phagocytes o If the bacteria are not encapsulated, macrophages can ingest the bacteria immediately  Capsule hide bacteria from recognition by macrophage receptors • Antibodies must coat the capsule, then the bacteria can be identified, then ingested