5. Antibody Structure and Effector Functions Flashcards
What is the basic structure of antibodies?
- 4 polypeptide chains
> 2 identical light (L) chains/ 2 identical heavy (H) chains - each chain has a variable (V) region/ constant (C) region
What region of antibodies defines it?
- constant regions
> 2x light chain subtypes> κ/ λ
> 5x heavy chain isotypes > υ/ γ/ α/ δ/ ε
What determines the class of antibody?
- isotype of constant region of heavy chain (υ/ γ/ α/ δ/ ε)
How is each light chain (outer) bound to its partner heavy chain (inner)?
- a disulfide bond
- non-covalent interactions
How are the 2 heavy chains (inner) brought together?
- disulfide bonds
- non-covalent interactions
What is the hinge region on antibodies?
- between CH1/ CH2
- gives flexibility to antigen-binding arms
What is Fab?
- fragment antigen binding region
> where antibody binds to antigen
What is Fc?
- fragment crystallizable
> interacts with FcRs that mediate antibody effector functions
What do the amino/carboxy-terminal domains of antibodies do?
- amino-terminal portion (Fab region) binds to antigen
- carboxy-terminal mediates effector functions (Fc region)
How are antibodies made up of multiple immunoglobulin domains?
- anti-parallel β-pleated strands arranged into a pair of sheets
> each β sheet contains 3-5 strands - loosely folded regions link the strands
What are CDRs?
- complementarity determining regions
> 3 loops of variable domain with high variation in AAs (hypervariable regions)
What are the 5 major classes of antibodies?
- IgG/ IgD/ IgE/ IgA/ IgM
What joins antibody monomers together?
- J chain (linked by 2 disulfide bonds)
> serum IgM- pentamer
> serum IgA- mostly monomer, can be dimer/ trimer/ tetramer
What antibody is always a pentamer?
- serum IgM
What antibodies are multimers?
- Serum IgM > always a pentamer
- Serum IgA > mostly monomer but can be dimer/ trimer/ tetramer
What determines the class of antibody?
- differences in AA sequences in heavy chain constant regions
> 5x isotype of heavy chain constant regions
What antibodies do not have a J chain?
- IgG/ IgD/ IgE
- only IgA/ IgM are multimers (J chain joins monomers together)
Why are there 5 major classes of antibodies?
- 5 basic sequences of heavy-chain constant regions (υ/ γ/ α/ δ/ ε)
- each different heavy-chain constant region is called an isotype
- the isotype of the heavy chains of an antibody determines its class
What heavy chain isotypes correspond with each class of antibody?
- υ > IgM
- δ > IgD
- γ > IgG
- ε > IgE
- α > IgA
What major classes of antibodies can be further sub-classified?
- α/ γ heavy chain isotypes further classified into sub-isotypes
- corresponding antibodies IgA/ IgG further classified into subclasses
- 2 sub-isotypes of α heavy chain: α1/ α2
> 2 IgA subclasses: IgA1/ IgA2 - 4 sub-isotypes of γ heavy chain: γ1/ γ2/ γ3/ γ4
> 4 IgG subclasses: IgG1/ IgG2/ IgG3/ IgG4
What is the only antibody that crosses the placenta?
> fetal circulation
IgG
What is the first antibody produced during a primary immune response?
IgM
What are antibody isotypes?
- antigenic determinants within constant regions of antibodies
> shared among all members of species (same for everyone)
What are antibody allotypes?
- antigenic determinants within constant regions of antibodies
- alternative allelic forms of same isotype
> vary among individuals (different for everyone)
What are antibody idiotypes?
- antigenic determinants within antigen-binding site (Fab) of antibodies > sequence of its CDRs
- characterize a unique antibody
What are the 6 main antibody effector functions?
- Neutralization > inactivate/ prevent binding to cells
- Agglutination > enhances neutralization/ pathogen clearance
- Opsonization > phagocytosis
- Complement activation > formation of MAC > kill microbe
- ADCC- (Antibody-Dependent Cell-Mediated Cytotoxicity) > NK cell-induced apoptosis
- Degranulation > triggers mediator release from granulocytes
What are neutralizing antibodies?
- antibodies that bind pathogens/ prevent them from binding to other cells
> can prevent pathogen from ever initiating an infection
What are the first antibodies produced during a primary immune response?
IgM
- low-affinity antibodies (since B cells that make them have not yet undergone affinity maturation)
- very effective at binding pathogen since decavalent (can bind 10 antigens) since are pentamers
What are the effector functions of IgM antibodies?
- fixing complement > lysis of pathogens they bind to
- agglutination of pathogens > phagocytosis by macrophages
What are known as “natural antibodies”?
- IgM antibodies produced by B-1 cells (most circulating IgM antibodies)
- B-1 cells constitutively produce IgM cells without being exposed to pathogen > protect from common gut/ mucosal pathogens
What is the most common antibody class in the serum?
- IgG antibodies (most abundant)
> also most diverse class (4 subclasses- IgG1 > IgG4)
What are the effector functions of IgG antibodies?
- all variants bind to Fc receptors
- opsonization > enhance phagocytosis by macrophages
What is the major antibody isotype found in secretions?
IgA
- mucus in gut/ respiratory and reproductive tracts/ milk from mammary glands/ tears/ saliva
What are the effector functions of IgA antibodies?
- neutralization of pathogens
- interact with commensal bacteria/ prevent from entering bloodstream
- IgA can not fix complement
- mediate ADCC by binding FcRs on NKT cells
- trigger degranulation of granulocytes
What are the effector functions of IgE antibodies?
- best known for role in allergy/ asthma
- protection against parasitic helminths (worms)/ protozoa
- induce degranulation of eosinophils/ basophils
What are the effector functions of IgD antibodies?
- minor immunoglobulin in blood (0.2%) > respiratory tract
- degranulation > bind to basophils/ mast cells > release antimicrobial peptides/ cytokines/ chemokines
What are FcRs?
- Fc receptors > on innate cells/ APCs (most immune cells)
- mediate effector functions of antibodies
Why are Fc receptors important/ how do they work?
- mediate effector functions of antibodies
- multiple FCRs must be cross-linked to initiate signal
> positive signal- enhances effector function
> negative signal- inhibits effector function - outcome depends on whether receptor associated with ITAM/ ITIM
> immunoreceptor tyrosine activation/ inhibition motif - most FCRs are activating receptors/ associated with ITAM
What is the most diverse group of FcRs?
- FcγR
- 4 families > 3 activating/ 1 inhibiting family of receptors
- main mediators of Ab functions in the body
What antibody effector functions are mediated by FcγR?
- main mediators of Ab functions in the body
- induce phagocytosis if expressed by macrophages
- induce degranulation if expressed by cytotoxic cells
What antibody effector functions are mediated by FcεR?
- expressed by granulocytes (mast cells/ basophils/ eosinophils)
- triggers inflammation cascade > release histamines/ proteases/ inflammatory mediators
> associated with allergy symptoms
What antibody effector functions are mediated by FcαR?
- expressed by myeloid cells
- trigger ADCC/ phagocytosis > contribute to pathogen destruction
- stimulate myeloid cells to release inflammatory cytokines/ generate superoxide free radicals > kill internalized pathogens
What is the neonatal Fc receptor?
FcRn
- expressed on epithelial/ endothelial cell types
- helps carry Ab ingested in milk across epithelial cells > bloodstream
- in adults > help recycle IgG taken up through endothelial cell pinocytosis
What antibody effector functions are mediated by PolyIgR?
- expressed by epithelial cells
- initiates transport of IgA/ IgM from blood to lumen of tissues in gastrointestinal/ respiratory/ reproductive tracts
> transports IgA across epithelial borders - carries antibody into secretions
- populates gut mucosa with IgA to protect against ingested microbes
