Flashcards in 03/06a Antibodies Deck (58):
What are the two forms of antibodies?
Membrane-bound on the surface of B cells
Secreted into circulation, tissues, and mucosal sites
What are the functions of secreted antibodies? List three
Prevent the entry and spread of pathogens
What is the general structure of antibodies?
2 identical heavy chains
2 identical light chains
Each chain has a constant region and a variable region (variable region is the antigen-binding domain)
What is the immunoglobulin domain?
The functional region of the antibody molecule
Lights chains have one variable region domain and one constant region domain
Heavy chains have one variable region domain and three or four constant region domains
What is an antigen?
Any substance that may be specifically bound by an antibody molecule or a T cell receptor
What is an antigenic determinant?
The specific shape or surface on a molecule that may be recognized by an antibody; also called an epitope
What is an immunogen?
An antigen that can activate lymphocytes
Small antigens may not be immunogenic because they cannot cross-link B cell receptors
What is a hapten?
A small antigen bound to a carrier, in order to become an immunogen
What epitope is recognized by T cells?
Short peptide fragments, based on their primary structure
What epitope is recognized by B cells and antibodies?
3-dimensional structure of any molecule
What are the two types of B cell antigens? Describe them and the responses they elicit
T-dependent antigens - most common response; usually proteins, cannot stimulate antibody production without the help of T cells
T-independent antigens - large polymeric molecules like polysaccharides, can stimulate multiple B cell receptors on the same cell, but have low affinity and poor memory generation
What is the complementarity determining region?
Amino acid loops found in the variable regions of both the heavy and light chains, that come together to form the antigen-binding pocket
Also called the hypervariable region
3 CDRs in each variable heavy chain and each variable light chain
What type of forces contribute to antigen-antibody binding?
NON-covalent forces like electrostatic forces, hydrogen bonds, Van der Waals forces, and hydrophobic forces
What is the affinity of an antibody?
The strength of binding between a single antibody arm and an antigen epitope
What is the avidity of an antibody?
The overall strength of attachment of an antibody to an antigen (antibodies are multimeric and can bind an antigen multiple times)
Much greater than the affinity of any one antibody-antigen interaction
What two factors determine avidity?
Valency - how many times a single antibody molecule can bind an antigen
Affinity - strength of binding between antibody arm and antigen epitope
What are the two functional regions of an antibody? What is their purpose?
Variable regions define the antigen-binding site
Constant regions (heavy chains) define the physiologic and biologic properties of the antibody
What does mercaptoethanol reduction do to an antibody?
Dissolves the disulfide bonds and yields separate heavy chains and light chains
What are the four major functions of antibodies?
1) Act as B cell receptors
2) Neutralize or block activity of pathogens and toxins
3) Allow recognition antigen-antibody complexes by Fc receptor-expressing effector cells (phagocytes, eosinophils and NK cells, other immune cells)
4) Activation of serum complement
How are B cell receptors activated? How do they initiate signaling?
Cross-linking by antigens
Signal through associated signaling molecules like Ig-alpha and Ig-beta (BCR structure does not include a signaling motif)
What do neutralizing antibodies do?
Bind to key epitopes on microbes and toxins, and block their binding to cellular receptors
Requires only the antigen-binding region of the antibody
Higher affinity antibodies produce better neutralization
Which vaccines depend on neutralizing antibodies to generate protection?
Polio and hepatitis (neutralization of virus)
Tetanus and diphtheria (neutralization of toxin)
What is agglutination?
Clumping of cells or particles brought about by antibody binding to antigens on different cells or particles, and subsequent cross-linking
Requires intact antibody
Clumping of cells/particles inhibits their activity and makes them more susceptible to phagocytosis
What is opsonization?
Coating of microbes by antibodies, which promotes phagocytosis
Mononuclear phagocytes and neutrophils express receptors for the Fc portion of antibodies and specificly bind clustered antibodies
What are Fc receptors?
Receptors on the surfaces of immune cells that recognize the Fc portion of antibodies
Specific for certain types of antibodies
Most have a low affinity for FREE antibodies - must be bound to something to initiating signaling (this allows for very specific attack by immune cells)
What are the functions of Fc receptors? List four
2) Negative feedback regulation, particularly for B cells
3) Induce killing of target cells by effectors (NK cells and eosinophils)
4) Activate cells such as mast cells and basophils to release granules
What is different about the Fc-epsilon receptor?
It has a very high affinity for free unbound IgE antibodies
Mast cells are constitutively coated with IgE
Antigen-binding to and cross-linking of IgE stimulates mast cells, basophils, and eosinophils to release their granules
What is the Fc-gamma receptor?
Binds to IgG
Found on macrophages, neutrophils, eosinophils, and dendritic cells
What is the Fc-gamma receptor III?
Mostly found on NK cells
What is antibody-dependent cell-mediated cytotoxicity?
Antibodies bind to and coat cells (such as virus-infected cells)
Fc-gamma receptor III on NK cells binds these antibodies and becomes cross-linked, activating the NK cells to release cytokines and cytotoxic granules
How do antibodies aid in the destruction of helminths?
Fc receptors on eosinophils bind to antibodies that coat a parasite
This causes eosinophil degranulation, releasing major basic protein and other granule contents to kill the parasite
Why do helminths require a specially toxic protein product to be destroyed?
They are too large to be engulfed by phagocytes, and are resistant to the microbicidal products of PMNs and macrophages
How do antibodies initiate the complement cascade?
Complement binds to the constant regions of IgG or IgM when they are bound to antigen, which initiates the complement cascade
What features are antibodies classes based upon?
Structural variations in the antibody heavy chains
What are the five major classes of antibodies in humans? How do they have different functions?
IgG, IgA, IgM, IgE, and IgD
IgG and IgA have subclasses as well
Their different functions are based on the structure of their Fc regions
What are the five classes of heavy chains in humans?
Gamma, alpha, mu, epsilon, and delta (correspond to antibody class letter)
Heavy chain defines the antibody isotype and subclass
What are antibody allotypes?
Variants in the constant regions of gamma, alpha, and kappa chains that arise from allelic variation between individuals
May confer some biological advantage against certain infectious agents
What are antibody idiotypes?
Epitopes associated with the antigen-binding region
An antibody of a given idiotype has ONE specificity
What are the three types of anti-antibodies? Describe what they bind to
Anti-isotype - binds to all antibodies of a given type (e.g. anti-human IgG)
Anti-allotype - binds to antibodies that share a genetic variant (antibodies from related individuals)
Anti-idiotype - binds to antibodies that have a specific variable region structure (ONE particular antibody clone)
How is antibody function determined?
Depends on antibody isotype
Sites in the Fc region control physiologic properties, including what tissues it can access, length of persistence, and what non-specific effector molecule or cells may be recruited
What are the functions of IgD?
Functions almost exclusively as a B cell receptor (membrane-bound, not found in serum)
Activates B cells and associates with Ig-alpha and -beta chains (adaptive molecules for signaling)
What is the structure of IgM?
Secreted form - pentamer with J chain (joining)
Membrane-bound form - monomer
What are the functions of IgM?
First antibody to be made in the fetus and by activated naive B cells
Efficient for complement activation and agglutination (high avidity for antigen)
Third most common serum immunoglobulin
Active in A/B blood group recognition
What is the structure of IgA?
Dimer with a J chain
Secreted version has secretory component
What are the functions of IgA?
First line of defense - the major class of Ig in secretions (saliva, tears, mucus, etc.) on mucosal surfaces
Can neutralize pathogens and agglutinate viruses to prevent their entry into cells
Plays a role in passive immunity - transferred to neonates from the mother in breastmilk
Second most common serum Ig
What are the subclasses of IgG? How do they differ from one another?
IgG1 through IgG4
Differ in the numbers and arrangement of their inter-chain disulfide bonds, which results in different functional properties
What are the functions of IgG?
Highly versatile - can carry out all antibody functions
Major antibody in serum and extravascular spaces
Agglutinates and precipitates pathogens
Opsonizes and neutralizes pathogens and activates complement
Can pass across the placenta
Functions in ADCC
What are the functions of IgE?
Binds very tightly to Fc receptors on basophils and mast cells, and activates basophils, mast cells, and eosinophils against parasitic helminth diseases
Also involved in allergic reactions
Does NOT fix complement
Least common serum Ig
What happens to the antibody response over time after exposure to a pathogen?
Starts predominantly with low affinity IgM
Progresses to higher affinity IgG
Affinity of the antibody clones for antigen will improve
How are neonates protected after birth? Why do they need this protection?
Major defense after birth is passive immunity that is provided by maternal antibodies
Neonates lack the ability to mount effective immune responses
What antibodies are involved in neonatal immunity? How are they transmitted from mother to baby?
Maternal IgG is transported across the placenta, and maternal IgG and IgA are ingested through breast milk
Ingested antibodies are active against pathogens in the gut, and can also enter the baby's circulation
What are the four ways in which antibodies can be used therapeutically?
Ig fusion proteins
What is antisera? How does it work?
Antisera is serum from immune individuals or antibodies that contains polyclonal antibodies against a specific substance
Conveys passive immunity
Commonly used as antitoxins or antivenoms
What are some common therapeutic antisera?
Rabies immune globulin - rabies treatment
Tetanus immune globulin - tetanus treatment
Heptavalent Botulinum Antitoxin (HBAT) - botulism treatment
What is intravenous immunoglobulin?
Polyvalent IgG from the plasma of many donors
Given intravenously to immunodeficient patients, for some autoimmune diseases, and to treat some rare acute infections
Contains IgG from all four subclasses
Lasts a long time
By what mechanisms is IVIG effective?
Antibodies possess very broad neutralizing activities
Anti-inflammatory effects (for autoimmune disorders)
May stimulate inhibitory Fc receptors on B cells and dendritic cells, leading to decreased cell activation
What are monoclonal antibodies?
Monospecific antibodies made from immortalized and cloned B cells, that bind to a single epitope
Used in research and diagnostics, and increasing in as therapeutics