CH. 4: Complement

Question 1

Describe complement.

Answer 1

• group of proteins that enhances ability of antibodies and phagocytic cells to clear pathogens from an organism
• primarily produced by liver; circulate in inactive form in blood
• when encountering pathogens or antibodies bound to pathogens, they undergo a cascade of reactions, activating each other to fight infection

Question 2

Activation pathways: Describe the classical pathway.

Answer 2

• activated when C1 binds to antigen-antibody complex
• IgM and IgG (esp IgG3) are effective activators
• IgM > IgG3 > IgG1&raquo_space; IgG2 (most to least effective to activate C1)

Question 3

Activation pathways: Describe the lectin pathway.

Answer 3

• activated when mannan-binding lectin (MBL) binds to terminal polysaccharide residues on surface of many types of microbes (gram-positive and gram-negative bacteria, fungi, or yeast)
• can be activated by ficolin (serum protein), which binds to acetylated molecules on microbial surfaces

Question 4

Activation pathways: Describe the alternative pathway.

Answer 4

• activated when C3b deposits on surface of pathogen

Question 5

Biological activities of complement: Describe opsonization.

Answer 5

• opsonization via complement receptors (CR1, CR3, CR4)
• bacteria coated by opsonins are rapidly taken up and destroyed by phagocytic cells

Question 6

Biological activities of complement: Describe cell lysis.

Answer 6

• formation of MAC on surface of a cell, which leads to lysis of cell (esp microbial pathogens)

MAC = “membrane attack complex”

Question 7

Biological activities of complement: Describe anaphylatoxins.

Answer 7

• C5a > C3a > C4a (most potent to least potent)
• induces inflammation, chemotaxis, and smooth muscle contraction

Question 8

Biological activities of complement: Describe other important functions.

Answer 8

• enhances B cell responses to antigen (CR2 receptor for C3d)
• enhances dendritic cells (C3d)
• antigen clearance – keeps immune complexes small making clearance by macrophages easier; helps attach small immune complexes to RBCs to assist in clearance
• removal of dead or dying host cells; dead host cells activate complement