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