Antibody Function II Flashcards
List the components of complement in the order in which they become activated in the classical pathway.
activated by complexes of IgG or IgM antibody with antigen. Fc portion of antibodies change which allows binding and activation of C1q. C1q must interact with the 2 Fcs simultaneously (2 IgGs close together or IgM). C1 activates C4 and then C2, together they activate C3, which activates C5-C6-C7-C8-C9. Activating C3 and C5 is a vital part of complement because they are responsible for opsonizing, chemotaxis and anaphylastoxic
Name those that are also activated in the alternative pathway.
activated by IgA-antigen complexes, bacteria may activate C this way even in the absence of antibody. This pathway is also part of the innate immune response. Complement is activated by C3 (always breaking own at a low rate to C3a and C3b). IgA and the cell wall structures provide a surface for binding of C3b (thus stabilizing it) and factor B, properin and factor D forms which can activate C5 (and thus C6-C7-C8-C9).
Describe how the lectin pathway of complement activation is triggered, and whether it is part of innate or adaptive immunity.
Lectin pathway is part of the innate immunity. Pathway is mediated by mannose-binding protein (MBO or MBL), a lectin (protein that binds carbohydrates). MBP binds certain mannose-containing structures found in carbohydrates of bacteria but not humans. MBP is similar to C1q so the lectin pathway goes MBP-4-2-3-5-6-7-8-9
Discuss the different ways in which complement is activated by IgG and IgM
C1q must interact with 2 IgG Fcs close together or a single IgM (reinforces that IgM is for efficient at activating complement than IgG)
Opsonizing:
C3b adheres to membranes, phagocytic cells have C3b receptors and so they get a firm grip on an antigen if opsonized with C3b. IgG is also opsonizing because phagocytes have receptors for its Fc end called FcR.
Lytic:
if the membrane attack complex (MAC) is activated when C5 activates C6-C7-C8-C9. C8 and C9 form a lesion on the target cell membrane which looks like a hold and the cell loses ability to regulate osmotic pressure and lyses or pops.
Anaphylatoxic:
C3a, C4a and C5a can all release histamine from mast cells by binding. This leads to increase blood flow to the area of antigen deposition and a better chance for inflammatory cells to get out of the blood and into tissues.
Chemotactic:
the C5 activation product, C5a, is chemotactic for phagocytes, especially neutrophils.
Discuss how complement is important in immunity to bacteria even if the bacteria are resistant to lysis by C9. Identify the family of bacteria for which lysis is necessary for their clearance.
Not all bacteria need to complete the complement pathway all the way to C9, many can be killed via opsonizing with just C5 activation. The most susceptible family of bacteria to lysis is Neisseria (gonorrhea and meningitis) which may require the activation of membrane attack complex.
