membrane attack complex

(noun)

The final complex of all complement system pathways that lyses the pathogen. It is composed of C5b, C6, C7, C8, and C9.

Related Terms

  • C5a
  • mannan-binding-lectin
  • innate immune system
  • complement system

Examples of membrane attack complex in the following topics:

  • Role of the Complement System in Immunity

    • The end result of this activation cascade is massive amplification of the response and activation of the cell-killing membrane attack complex (also called MAC).
    • C5b forms a complex with C6, C7, and then C8, and C9, which becomes the membrane attack complex that lyses the pathogen.
    • This factor prevents membrane attack complexes from forming on the body's cells under normal conditions.
    • Similarly to the classical pathway, C3b forms a C42b complex, and the rest of the steps are essentially the same as the classical pathway, ending with C5b forming a membrane attack complex with C6, C7, C8, and C9.
    • The classical and alternative complement pathways start off differently, but end in the same cascade of complement proteins that combine to form a membrane attack complex.
  • Types of Adaptive Immunity

    • Cytotoxic T cells kill pathogens in several ways, including the release of granules that contain the cytotoxins perforin and granzyme, which lyse small pores in the membrane of a pathogen.
    • They also neutralize the toxins produced by certain pathogens and provide complement pathway activation, in which circulating proteins are combined in a complex cascade that forms a membrane attack complex on a pathogen cell membrane, which lyses the cell.
  • WBC Function

    • The antibodies bind to pathogens to opsonize (mark) them for phagocytes to engulf, neutralize them, or start a complement cascade in which proteins form a membrane attack complex to lyse the pathogen.
  • Structure and Function of Antibodies

    • Using this binding mechanism, an antibody can neutralize its target directly or tag it for attack by other parts of the immune system.
    • The complement system starts a long cascade of protein productions that either opsonize a pathogen for phagocytosis or lyse it directly by forming a membrane attack complex.
  • Major Histocompatibility Complex Antigens (Self-Antigens)

    • Major histocompatibility complex (MHC) is a cell surface molecule encoded by a large gene family in all vertebrates.
    • MHC III- a secreted enzyme that is not membrane bound, nor involved in antigen presenation like MHC I and II.
    • Organ transplantation is a complex procedure that can potentially cure many chronic diseases or damaged conditions.
    • If the T and B cells of the body recognize the HLA of the graft as foreign, they will attack the organ graft.
    • MHC-I/peptide complexes enter Golgi apparatus, are glycosylated, enter secratory vesicles, fuse with the cell membrane, and externalize on the cell membrane interacting with T lymphocytes.
  • Hypersensitivity

    • Associated disorders: Autoimmune hemolytic anemia, thrombocytopenia, Erythroblastosis, fetalis, Goodpasture's syndrome, Membranous nephropathy, Graves' disease, Myasthenia Gravis.
    • Immune Complex Hypersensitivity (Type III) is an immune response to immune complexes (antigen-antibody complexes) that become deposited in a given area.
    • This results in continuous localized immune attack (and ensuing tissue damage).
  • Regulation of Blood Cholesterol Levels

    • However, the different lipoproteins contain apolipoproteins, which serve as ligands for specific receptors on cell membranes.
    • Now within the cell, the cholesterol can be used for membrane biosynthesis or esterified and stored within the cell, so as to not interfere with cell membranes.
    • These plaques are the main causes of heart attacks, strokes, and other serious medical problems, leading to the association of so-called LDL cholesterol (actually a lipoprotein) with "bad" cholesterol.
    • Lipid vesicles (or liposomes) are small, intracellular, membrane-enclosed bubbles of liquid within a cell.
    • They are formed because of the properties of lipid membranes, where the hydrophobic chains are packed together so they are not in contact with the aqueous solution.
  • Overview of Adaptive Immunity

    • This mechanism allows the immune system to mount stronger attacks each time the pathogen is encountered, thus preparing itself for future challenges and preventing reinfection by the same pathogen.
    • An antigen is any molecule that induces an immune response, such as a toxin or molecular component of a pathogen cell membrane, and is unique to each species of pathogen.
    • The antigen is processed by the APC and bound to MHC class II receptors and MHC class I receptors on the cell membrane of the APC.
    • Cytotoxic cells directly attack cells carrying certain foreign or abnormal molecules on their surfaces.
    • In most cases, T cells only recognize an antigen if it is carried on the surface of a cell by one of the body's own MHC, or major histocompatibility complex, molecules.
  • Lymphoid Cells

    • NK cells contain receptors for a molecule called MHC (major histocompatibility complex) class I, which allows the NK cell to distinguish between infected cells and tumors from normal and uninfected cells.
    • Normal cells express MHC class I on their cell membranes, while infected or cancerous cells do not express or express reduced amounts of the molecule.
    • Activated NK cells release cytotoxic (cell-killing) granules that contain perforin and granzyme, which can lyse (break down) cell membranes and induce apoptosis to kill infected or abnormal cells.
    • Sometimes these lymphocytes react to antigens that aren't harmful (allergy) or will attack antigens expressed from the host's own body (autoimmunity).
  • Direct Gene Activation and the Second-Messenger System

    • In the absence of ligand, type II nuclear receptors often form a complex with co-repressor proteins.
    • The G-protein is bound to the inner membrane of the cell and consists of three sub-units: alpha, beta, and gamma.
    • Upon binding to the receptor, it releases a GTP molecule, at which point the alpha sub-unit of the G-protein breaks free from the beta and gamma sub-units and is able to move along the inner membrane until it contacts another membrane-bound protein: the primary effector. 
    • General schematic of second messenger generation following activation of membrane bound receptors. 1.
    • The agonist activates the membrane-bound receptor. 2.
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