Lecture 7: Molecular Basis Of Antigen Recognition Flashcards
There are 2 membrane bound lymphocyte receptors: ______ and ______. There is one type of soluble receptor, which are ___________
BCRs; TCRs
Antibodies
Antigen presenting molecules are MHC types I and II, also known as ______ types I and II
HLA
On what cells would you find class I HLAs?
All nucleated cells
On what cells would you find class II HLAs?
Professional APCs (dendritic cells, macrophages, B cells)
What are 6 major members of the immunoglobulin superfamily?
IgG TCR Class I MHC CD4 CD28 ICAM-1
[members of the same family due to similarities in domain structure, function, and evolution]
__________ = the part of a molecule that is recognized by the immune system and is loosely associated with adaptive responses
Antigen
________ = an antigen that evokes a specific immune response
Immunogen
True or false: all immunogens are antigens
True; note that not all antigens are immunogens
________= an antigen that induces immunologic tolerance
Tolerogen
__________ antigens are the body’s own cellular components or intracellular pathogens
Endogenous
Endogenous antigens can be further classified into:
_______= self-antigens
_______= tissue-specific antigen, which is present in one individual of a species but not in others (ex: ABO, HLA, etc.)
_______ = viruses, IC bacteria, and parasites
Autoantigens
Alloantigens
Intracellular pathogens
__________ antigens = antigens that enter the body or system and freely circulate in fluids and are trapped by APCs. The uptake of these antigens by APCs are mainly mediated by phagocytosis
Exogenous
A ________ is a small molecule which cannot induce an immune response on its own
Hapten
Under what condition can a hapten induce an immune response?
When coupled with a carrier protein
What are matters of clinical importance concerning haptens?
Drug allergies (penicillin itself is a hapten - not enough to elicit a response, must first bind to host proteins. IgG antibody may bind drug on RBC surface and cause hemolysis)
Vaccine design
The BCR is composed of a surface __________ and two invariant chains (______ and ______)
Immunoglobulin
Ig-Alpha; Ig-beta
What is the role of the Ig-alpha and Ig-beta complexing with the BCR?
They ensure surface expression of the immunoglobulin and function in signal transduction
[the BCR alone cannot participate in cell signaling–does not have ITAMs on cytoplasmic tails]
Circulating Abs are soluble ________ that recognize and bind Ags.
They promote killing and/or removal of the immune complex formed through the activation of effector mechanisms.
They also function as membrane-bound surface Ag-receptors on B cells and play a key role in B cell ____________.
Glycoproteins
Differentiation
Antibodies consist of _____ polypeptide chains
4
What is the basic structure of an antibody?
2 Heavy chains (each with variable and constant regions)
2 light chains (each with variable and constant regions)
The variable regions of the Ab are also called the ____ region
Fab
The constant regions of Abs are also called ____
Fc
Which structural chain of the Ab determines its class? (Like IgA, IgM, etc.)
The constant region of the heavy chains
All antigen binding occurs in what region of the Ab?
Fab region (the variable regions of both chains)
What is the effect of papain digestion of an Ab?
Cleaves the Ab in two places, separating the two Ag binding regions (now have 2 Fabs that can each bind in monovalent manner), leaving the Fc portion complete
What is the effect of Pepsin digestion of an Ab?
Cleavage and degradation of Fc portion of Ab, leaving behind the Fab portion (now called F(ab’)2) which maintains its bivalent binding capability
Basically chops off the tail of the Y, leaving behind the V
Each variable domain of an Ab has ____ hypervariable regions flanked by ____ framework (FR) regions
3; 4
True or false: The hypervariable regions of Abs are located on 3 of the loops located farthest from the constant region
True
The framework regions of antibodies correspond to the _____ sheets and the remaining loops
Beta
What is the primary purpose of the hypervariable region of an Ab?
Intimately involved in Ag binding by creating an interaction site that is:
Complimentary in shape, charge, and hydrophobicity to the epitope that it binds
What region of the antibody increases flexibility so that an Ab can bind to Ag epitopes that are close together or far apart?
Hinge
In what 4 ways do the different classes of Ig differ?
Size
Charge
AA sequence
Carbohydrate content
[there are both structural and functional differences]
Each Ig isotype has its own _____. There are ____ isotypes in humans.
Gene; nine
___________ refers to allelic differences in the heavy chains of Abs
Allotype
[we all have IgG isotypes, but these may have subtle differences from person to person]
________ refers to antigenic determinants on the V regions of antibodies
Idiotype
[we may all see an antigen, but we respond with slightly different V region determinants]
Which Ig isotypes exist as monomers?
IgG
IgD
IgE
Which Ig isotype exists as a dimer?
IgA
Which Ig isotype exists as a pentamer?
IgM
Which Ig isotype is produced in the primary response to antigens, as well as the first Ab produced in neonates?
IgM
IgM makes up ______% of total serum Abs. It is a pentamer, so it has ______ Ag binding sites.
This makes it highly efficient for binding Ags with multiple repeating epitopes like ______ and _______, in addition to efficient binding of ____________
It can be expressed on B lymphocytes as a ___________
5-10%; 10
Viruses; RBCs; classical complement
Monomer
The ______ chain consists of an Fc linked polypeptide disulfide-bonded to 2 of the 10 mu chains.
J
Which Ig isotypes have J chains?
IgA
IgM
[mnemonic = JAM]
What is the purpose of the J chain found on IgM and IgA?
Binds to secretory cells to cross mucosa
What is the predominant Ab of secondary immune responses?
IgG
Which Ig isotype is a constituent of secondary immune responses and its secretory form functions in mucosal immunity?
IgA
Which Ig isotype is the most abundant class in the serum?
IgG
How many subclasses of IgG are there? How are they different?
4: IgG1-4
IgG1 is in highest concentration, IgG4 is least
Differences are on the H chain, and there are other subtle structural differences as well as important functional differences
Which Ig isotype is able to activate the classical complement pathway?
IgG or IgM
Which Ig isotype crosses the placenta?
IgG
IgA makes up _____% of total Ab in serum, and is usually in ___________ form but can be a dimer via J chain link
10-15
Monomer
Secretory IgA functions in mucosal immunity. Where is it typically found?
Colostrum
Saliva
Tears
Mucus
The __________ component of IgA is a polypeptide produced by epithelial cells of mucous membranes that binds to Fc domains of IgA dimer
Secretory
What is the significance of secretory IgA?
Present at entry point for antigens
Efficiency as a polymeric Ig isotype (dimer)
Newborn immunity (breastfeeding)
What Ig isotype is a constituent in secondary immune responses and is in lowest concentration in the serum?
IgE
IgE binds to blood _______ and tissue ______ cells by Fc receptor with very high affinity.
Basophils; mast
[via CD23a and CD23b]
IgE plays a role in what powerful pharmacologic reactions?
Asthma, allergies, helminth infections
Which Ig isotype makes up less than 1% of serum Ab concentration and has no known function in the serum?
IgD
A ________ form of IgD is present as an Ag-specific receptor on mature B cells.
A B cell may express monomeric IgD and monomeric _____ as their receptor, though both of these will have the same Ag specificity.
Monomeric
IgM (mIgM)
True or false: ligation of mIgD leads to activation of B lymphocytes
False! Because they do not have an Ig-alpha + Ig-beta complex with them
Abs bind Ags via _________bonds
Non-covalent
[like hydrogen bonds, electrostatic bonds, Van der Waals forces, hydrophobic forces]
The binding between Abs and Ags is _______ compared to covalent bonding, but together generate a _______affinity interaction
Weak; high
True or false: Ab/Ag binding is reversible
True
The strength of the interaction between a univalent Ag (epitope) and univalent Ab (idiotope) is loosely referred to as ________
Affinity
The strength of the interaction between a multivalent Ag and a multivalent Ab is referred to as its ________
Avidity
The avidity of an Ab for its Ag is dependent on the _________ of of the individual Ag combining sites
Affinities
Epitopes generally contain __________ amino acids. The epitope may consist of either sequential or non-sequential amino acids.
If non-sequential, the 3D conformation of the epitope is vital; it may consist of complex proteins containing multiple overlapping epitopes.
Epitopes may be associated with _______ immunogens or ________ immunogens
Hydrophilic
Soluble; particulate
Define T-dependent Ags
Ags that require both T helper cells and B cells to stimulate and Ab response
T-dependent Ags are PROTEINS
Define T-independent Ags
NON-PROTEIN Ags, such as polysaccharides and lipids that can stimulate antibody responses WITHOUT T cell help.
Usually multiple identical epitopes that can cross-link several BCRs.
Like the B cell receptor, TCRs cannot participate in signaling by themselves. What other components of the TCR complex participate in signaling?
CD3 complexes (epsilon/gamma and epsilon/delta) = 4 total ITAMs
Zeta chains = 6 total ITAMs
[overall 10 ITAMs for T cell signal transduction]
Most TCRs exist as a heterodimer - they consist of an alpha and a beta chain. Each chain has a ________ region and a _______ region
Variable; constant
The ______ region of the TCR contains 3 hypervariable regions or CDRs. CDR ___ is the most hypervariable.
Variable
3
[CDR3 exists at the center of the molecule]
The majority of T cells are alpha/beta, what is the other type of T cell and where are they found?
gamma/delta - found in epithelial and mucosal compartments, recognize lipid Ags, recognize DAMPs, do not seem to be MHC restricted
[make up 5-10% of the T cell population]
What co-receptor is found with helper T cell TCRs?
CD4 (one chain consisting of alternating V-H-V-H domains)
[helps other cells of the immune system by activating macrophages for phagocytosis, secretion of cytokines, and stimulation of B lymphocytes to produce Abs]
What co-receptor is found with CTL TCRs?
CD8 (alpha chain and beta chain)
[functions to kill infected cells and transformed cells]
TCRs recognize primary sequences of amino acids in ____________.
Proteins
Are free peptides recognized by TCRs?
NO
The antigenic determinants are limited to those portions of the Ag that can bind to _____ molecules. This is why there can be differences in the responses of different individuals.
MHC (HLA)
Superantigens are not processed, but bind directly to MHC ____ molecules and ____ region of the TCR
Class II
V-beta
[each SAg activates a distinct set of V-beta expressing T cells, depending on which V-beta gene segment the T cell is expressing.
The diseases associated with exposure to SAgs are, in part, due to hyperactivation of the immune system and subsequent release of biologically active cytokines by activated T cells.
What are some diseases related to superantigen activity?
Food poisoning (staph enterotoxins)
TSS (staph)
Scaled skin syndrome (staph)
Shock (strep pyrogenic exotoxin)
True or false: bacterial SAgs are the best studied but there are SAgs associated with viruses and other microorganisms as well
True
_________ antibodies are monospecific identical Abs that are produced by one immune cell. The immune cells are all clones of a single parent cell
Monoclonal
What are some uses for monoclonal antibodies?
Prevention of transplantation rejection
Metastatic cancer (colorectal and H&N)
Diabetic retinopathy
Hematologic cancer
Rheumatoid arthritis
Crohns disease
Describe antigen recognition by B cells in terms of:
Interaction with Ag:
Binding of soluble Ag?
Involvement of MHC molecules?
Chemical nature of antigens:
Epitope properties:
Interaction with Ag involves binary complex of membrane Ig and Ag
Binding of soluble Ag = YES
Involvement of MHC molecules = NONE required
Chemical nature of Ags = protein, polysaccharide, lipid
Epitope properties = accessible, hydrophilic, mobile peptides containing sequential or nonsequential amino acids
Describe antigen recognition by T cells in terms of:
Interaction with Ag:
Binding of soluble Ag?
Involvement of MHC molecules?
Chemical nature of antigens:
Epitope properties:
Interaction with Ag: involves ternary complex of TCR, Ag, and MHC
Binding of soluble Ag = NO
Involvement of MHC molecules = required!
Chemical nature of antigens = mostly proteins
Epitope properties: internal linear peptides produced by processing of Ag and bound to MHC molecules
