Complement Flashcards
What is complement?
group of serum and cell surface proteins activated by factors such as the combination of antigen and antibody that results in the generation of enzyme cascades leading to a variety of biological consequences
What are the three pathways that initiate the complement cascade?
classical, lectin, and alternative
When is the classical pathway initiated?
when IgM and IgG binds to antigen
What does the binding of IgM and IgG to the antigen in the classical pathway allow for?
the conformational change of the antibody to the Fc portion to allow for C1 to bind
When is the binding site open in the classical pathway?
only when the antibody is bound to the antigen
Would you classify the classical pathway as innate or adaptive?
both: the innate arm (complement) is working with the adaptive system (antibody)
What is the lectin pathway initiated by?
soluble pattern recognition molecules; specifically mannose binding lectin
Is the lectin pathway innate or adaptive?
innate
What is the alternative pathway initiated by?
the binding of C3b onto the cell wall and then another complement protein
What can C3b bind to?
the cell wall of bacteria, fungi and some viruses
What is the most important molecule to initiate the complement cascade?
C3b
Is the alternative pathway innate or adaptive?
innate
What do all pathways of the complement cascade result in?
C3 convertase enzyme and the formation of the membrane attack complex
Why is complement fixation by the classical pathway not occuring in the blood stream even though the antibody and complement components are circulating together?
the C1 binding site is not available becauase the antibody is not bound to the antigen; no conformationial change to the antigen to allow for binding
What are the classes of antibody that can fix complement?
IgM and IgG
Which antibody is most efficient at fixing complement and why?
IgM; because only IgM molecule is required to activate the complement, two IgG molecules are required to activate complement
What is the membrane attack complex?
a structure that is assembled in membranes after the activation of the complement cascade; hydrophilic on the inside and hydrophobic on the outside
What is the membrane attack complex composed of?
predominantly of 10 to 16 molecules of C9 that forms a barrel shaped pore
What does the membrane attack complex do?
it punches a hole in the target membrane so that it can directly lyse and kill the microbe
Why is the alternative complement pathway not initiated in mammalian cells but it is in bacterial cells?
because mammalian cells have protective mechanisms that inactivate C3b, bacteria do not
What are the four ways that complement can contribute to eliminating bacteria?
opsonization, increased vascular permeability, anaphylatoxins, neutrophil chemotaxis
What molecules are involved in opsonization?
C3b and the antibody IgG
What is opsonization?
the coating of bacteria which makes it easier for phagocytic cells to attach to and phagocytize the bacteria
What molecules are involved in increased vascular permeability?
C2a, C3, and C5a
