Antibody structure Flashcards
What is an antibody?
secreted form of the immunoglobulin produced by a B cell.
What are immunoglobulins?
antigen-binding molecules of B cells
Antibody molecules are identical to the ______ component of the B cell receptor
immunoglobulin
What are antibodies produced by?
plasma cells
T or F: each antibody produced by a single B cell has the exact same specificity for antigen
true
What is the clonal hypothesis?
There is an enormous repertoire of B cells that are produced in bone marrow- each B cell displays many copies of the same BCR on its surface.
During infection, lymphocytes with receptors that recognize the pathogen are activated and proliferate.
A stimulated B cell gives rise to ____.
antibody-secreting plasma cells
What is the basic structure of antibody molecules?
two identical copies of a heavy chain
two identical copes of a light chain
Describe the regions of an antibody.
Variable region- area that recognizes antigens
Constant region- very few differences
What stabilizes immunoglobulin domains?
disulfide bonds
What binds the light chains to the heavy chains?
disulfide bond
What is the Fc receptor?
receptors found on different phagocytes that can recognize different antibody isotypes
Also where complement binds
What are structural characteristics of the light-chain V domain?
Made of alternating beta sheets
Beta-barrel arrangement is stabilized by interchain disulfide bond.
Sequence of beta-chain is almost always the same
Where does most variability of light chains exist?
loop regions (hypervariable regions)
Structural characteristics of light-chain C domain?
Beta-sheets.
Interchain disulfide bond.
Sequence is constant, loops mostly constant
How many hypervariable regions are there in light chains?
3 (1,2, and 3)
What are hypervariable regions separated by?
framework regions (1-4)
What do framework regions encode?
alternating beta sheets of light chain molecule
What is an antigen?
any molecule that is bound by an antibody or a T cell receptor
What is an epitope or antigenic determinant?
portion of an antigen that is bound by antibody molecule
Antibody binding is _____ and _____.
non-covalent and reversible
What is antibody binding mediated by?
electrostatic forces
hydrophobic interactions
van der waals forces
hydrogen-bonding
What is affinity?
binding strength
Antibodies have (few/many?) restrictions with respect to binding.
few
What can antibodies bind to?
Native or denatured proteins, peptides
Other types of molecules (carbs, nucleic acids, small molecules, chemicals)
Soluble or particulate antigens.
What do T cells (always) recognize?
peptides
What is a linear epitope? discontinuous epitope?
linear- string of amino acids in a row
discontinuous- formed by tertiary structure of protein
T cell receptors only recognize ____ epitopes
linear
What is a multivalent antigen?
An antigen with many copies of an epitope or multiple epitopes
What are the roles of antibodies?
membrane bound: specific antigen receptors for B cells
secreted: bridge btw acquired and innate immune system.
What are functions of secreted antibodies?
mark microbes or infected cells for destruction by phagocytes (opsonization) or NK cells (antibody-dependent cell-mediated cytotoxicity)
Activates complement pathway
What is neutralization?
antibodies bind to pathogens and toxins and hinder them from interacting with host cells
What is the most abundant isotype?
IgG
What is the major difference between the subtypes of IgG?
the hinge region
How many constant and variable regions are in the IgG heavy chain?
3 constant, 1 variable
What is altered during isotype switching?
only the constant region of the heavy chain
What do differences in the constant chains of Ab isotypes allow for?
versatility of anti-body mediated immune response
What is different structurally in IgM and IgE from the other isotypes?
Have 4 constant regions and no hinge region
IgM is always secreted as a ____
multimer (a pentameric form)
What are the 5 copies of the immunoglobulin in IgM joined by?
J chain
Each IgM molecule has (how many?) identical binding domains?
10
IgA can be secreted as a _______
multimer (dimeric most important)
What joins dimeric IgA?
J chain
Which isotype has the longest half-life?
IgG
Why are serum levels of IgE extremely low?
Rapidly bound up by mast cells
Which isotype has the highest molecular weight?
IgM (b/c it is secreted as a pentamer)
Which isotypes are important neutralizers?
IgG and IgA
Which isotype(s) are important in opsonization?
IgG
Which isotype functions in sensitization of mast cells?
IgE
Which isotype is the most efficient activator of complement cascade?
IgM
Which isotype most efficiently transports across epithelium?
IgA
Which is the only isotype that transports across the placental barrier?
IgG
Which isotype is the most important in diffusion into extravascular sites?
IgG
What are the major functional regions of Abs?
Fc regions- binds to complement proteins and Fc receptors
Fab- contains antigen-binding sites (variable region)
