Ab effector functions
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What Ab isotypes are good at neutralization?
IgG1, IgG2, IgG3, IgG4, IgA
What Ab isotypes are good at opsonization?
What Ab isotypes are good at activation of a complement system?
IgM, IgG1, IgG3
- pentamers, confined to blood (complementation)
- low affinity (due to no somatic hypermutations)
- high avidity
- rapidly clears organisms from bloodstream
- principal Ab in blood and extracellular fluid
- sm, so diffuses out of blood into tissues (opsonize and complement)
- maternal IgG across placenta
What facilitates movement of maternal IgG to fetus?
FcRB protein carries IgG from endothelium to extracellular and maternal to fetal; looks like MHC-1; 2 FcRB/IgG
- principal isotype in secretions; dimer
- in blood as a monomer
- protects epithelial surfaces from infectious agents
- neutralizes toxins at mucosal sites
What is the structure of IgA?
dimer; uses J chain to hold dimer together in secretions
w/ mast cells below skin and mucosa and along blood vessels in epithelial and CT
- bound to IgE via high aff rec for Fc portion of IgE
- activated when Fc receptor bound IgE is cross-linked by binding of multivalent Ag
- IgE-mast act important in local inflammatory responses (causes granules to release contents to extracellular env to cause expelling of path..cough, sneeze, etc)
- made by B cells
- initiates polymerization
- covalently linked to Ab
- done by IgA and IgG
- Ab binds to toxin so that the toxin cannot bind to a cell receptor; Ab +toxin binds to macrophage instead
Ab coat surface of pathogen and facilitates ingestion of pathogen by neutrophils and macrophages
receptors for Fc region of Ab expressed on surface of phagocytes
on certain phagocytes to bind Fc part of Ab w/ pathogen attached
include Ab L chain domains
system of plasma prot that interact w/ components of innate and adaptive immune system to aid in elimination of pathogens
FDCs and Fc receptors
FDCs have Fc and complement receptors but DO NOT participate in phagoctytosis; only regular dendritic cells do
What are the main functional consequences of complement activation?
- 1. opsonization of pathogens
- 2. recruitment of inflammatory cells
- 3. direct killing of pathogens
What are the "early" events of the complement pathway?
proteolytic steps that generate C3 convertase that binds to pathogen surface via C1 protein
What does C3 convertase do?
- C3 convertase + pathogen= lg amts of C3b
- generates several sm fragments that serve as inflammatory mediators and C5b prot
What does C3b do?
binds to complement receptors on phagocytes (opsonization)
What does C5b prot do?
initiates "late" events of complement cascade
What are the "late" events of the complement cascade?
seq of polym rxns that result in formation of membrane attack complex (MAC) that directly damages membranes of certain pathogenes
How is the host cell protected from MAC?
- prot CD59 disrupts formation of pores in host
- DAF prot disrupts C3 convertase in host
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