Lecture 5: Adaptive Antigen Rexognition Flashcards
Activation of B cells
B-cells do not need an APC for them to become activated. Instead, they can bind soluble or cell-surface assx forms of antigens.
When a B-cell is activated, they can become a plasma cell (which secretes antibodies) or a memory cell.
Where are plasma cells found?
NOT IN PLASMA!
In lymphoid organs.
Activation of T cells
In order to activate T-cells, the antigen must be presented to them by a APC within a MHC.
Remember; the antigen must be a linear peptide fragment
B-cell receptors: Tell me about them
Similar or different to TCR?
What is their NH terminus like?
B-cell receptors are structurally similar TCR.
-The -NH-terminus of heavy and ligt chains is highly variable, as is the Va and Vb of T-cells.
In the constant region, there is limited variabled.
In a single B cell or T cell, BCR and TCT are identical.
BCR
- What is the antigen?
- Diversity?
- Signaling?
- Effector functions?
Anntigens- macromolecules
Diversity: Each clone has unique specificity
Signaling is inited by [Igalpha and Igbeta proteins]
The effector functions are mediated by the constant regions of the antibodies.
T Cell Receptor
- What forms of antigens does it recognize?
- Diversity?
- Signaling functions are mediated by?
- Effector functions?
- Linear, short sequences of AA presented on a APC.
- Each clone has unique specicity
- CD3 and ς
- No effector functions
What is another word for antibodies?
Immunoglobins.
Most antibodies are found in the _____ slowest migrating group of globulins, named _____ globulins.
Third
gamma
What effects do antibodies have?
Antibodies can:
- Organize T-cells
- Opsonization
- Activate compliment system
- Neutralize toxins
- Have direct anti-bacterial activity
- Immunodulation
What is the structure of antibodies?
Anitbodies have 4 polypeptide chains: two light chains and 2 heavy chains.
The chains are bound together by disulfide bridges and non-covalent interactions
H and L chains are divided into: variable (V) and constant (C) regions.
- Light chain has 1 variable and 1 constant regions
- Heavy chain has 1 variable region and 3 constant regions
Antigens bind on the variable regions.
C region determines the fate of the Ag.
Anitbodies can be cleaved to make 2 Fab fragments and 1 Fc fragment
Antbodies can be cleaved to create, what?
2 Fab fragments, where the antigen binds
1 Fc fragment, responsible for effector functions
We mentioned that antibodies can be cleaved. What happens they are cleaved by Papain?
Papain will cleave Ab into
2 Fab fragments
Fc fragment
We mentioned that antibodies can be cleaved. What happens when they are cleaved by Pepsin?
Pepsin will not seperate the two Fab fragments (F(ab)2. Thus, it will create a single bivalent antigen binding fragment.
How do we create different types of antibodies?
There are 5 different types of heavy chains (___, __, __, ___, alpha), which determine the class of the antibody.
There are also two kinds of L chains (k and __).
1 AB can only have one ____ or ___, but not both.
k or __-L chain
How do we connect L and H chains?
Disulfide bridges will connect them every 90 AA, creating a polypeptide loop.
These loops are referred to as VH, VL, CH1, CH2
What are the Ig superfamily proteins?
Cells surface and soluble protein involved in the [recognition, binding or adhesion] processes of cells.
- TCR
- MHC molecules
- CD4 receptor of T cells
- CD28
- costimulatory receptor on T-cells
- adhesion molecule ICAM-1
How are secreted IgG different from membrane bound IgM?
Secreted IgG: the heavy chain C-regions end in TAIL PEICES.
Fc receptor/compliment binding sites are approcimated.
Membrane bound IgG: has one more CH4 domain and is anchored into the cytoplasm.
How are antibodies so flexible?
Antibodies have hinges in between CH1 and CH2 domains.
How are the constant chains are from one another is determined by these hinges.
What are the 5 major classes of antibodies?
- IgA
- IgD
- IgE
- IgG
- IgM
IgA
Types?
Heavy chains?
Secreted forms?
Functions?
Remember; antibodies are classified based on their heavy chain and each may only have a k or delta light chain.
There are two types: IgA1 IgA2
Heavy chains are: alpha1 and alpha2
Secreted mainly as dimers, but also monomers and trimers.
Functions in [mucosal immunity].
IgD
Main function
Naive B cell antigen receptor
IgE main function
Defend against parasites and immediate hypersensitivity
IgG main function
- Opsonization
- Compliment activation
- Antibody dependent cell-mediated cytoxicity
- Neonatal immunity
- Feedback inhibition of B cells
IgM function
Naive B cell antigen R
Compliment activation
Which antibody is released as a pentamer?
IgM
The tightness of antibody-antigen binding is called affinty.
Do antibodies formed in the primary response, soon after infection of an antigen, have higher or lower affinity?
Lower.
Antibodies produced by a memory response have HIGH affintiy.
When is the affinity of the ab and antigen CRITICAL?
When the antigen is a toxin and needs to be neutrilized quickly at low titers.
What is antibody valence?
Antibody valence is the number of antigens a antibody can bind.
What is the valence of IgG?
IgG has 2 Fab regions; thus it can bind 2 molecules of an antigen OR 2 identical sites on 1 particle
Thus; valence= 2
How is valence related to binding affinity?
Increase valence; increase binding affinity
What is avidity?
Avidity is the overall strength of the Ab-Ag complex. It depends on
- Affinity
- Valence of both Ab and Ag
What is the affinity of IgM?
IgM has low affinity.
This is useful when we microogranism with a large number of binding sites.
Ben got infected with a microorganism with a LARGE number of binding spots. Which antibody is the most effective in killing dat shit.
IgM, which has low affinity.
Which has the highest avidity?
Monovalent antibodies
Bivalent antibodies (IgG)
Polyvalent antibodies (IgM)
IgM> IgG> monovalent antibodies.
Although IgM has low affinity, it has a high valence; meaning that its avidity will be HIGH.
How do we make monoclonal antibodies?
- Spleen cells from a mouse that have been immunized are isolated.
- Polyethylene glycol is then used to combine [isolated spleen cells] and [immortizalized myeloma cells that do not secrete antibodies].
- Cells are placed on a selection medium (HAT) that only allows immortilized cells to survive. HAT includes (hypoxanthine, aminopterin, and thymidine)
- Screen for anti-X anitbody and expand the + clones
Why do only hybrid cells survive in a HAT medium?
Because myeloma cells do not have HGPRTransferase that is used to purine salvage.
The aminopterin will block purine synthesis.
THUS; PURINES (DNA) CANNOT BE MADE :)
What is the basic structure of TCR?
T-cell receptors are heterdimeric (1 alpha and 1 beta chain).
Each chain has a variable (V) region and a constant (C) region (same as in antibodies).
The v region has 3 complemtarity-determining regions (CDR) that each form a loop.
Describe the TCR Complex
TCR is a heterodimeric protein with an alpha and beta subunit.
Alpha and beta subunits both have variable and constant regions.
The variable regions have 3 complimentarity determining regions (CDRs)
TCR will form a complex with CD3 and _ proteins due to charged areas in the transmembrane region.
The alpha and beta subunits of the TCR have tails that go into the cytoplasm that are 5-12 aa long. However, they cannot transduce signals. SO. CD3 and __ serve to transduce the signals for the TCR complex.
Structure of CD4 co-receptor
- Has 4 extracellular-antibody like domains.
- hydrophobic transmembrane region
- Basic tail with 38AA
Structure of CD8 receptor
2 chains:
CD8alpha
CD8beta
CD8 is made up of 2 related chains: CD8a and CD8b. What are characteristics of them?
- they have a single extracellular Ab domain
- hydrophobic transmembrane region
- basic cytoplasmic tail that is about 25 AA long
CD8 binds to class I MHC, which interact with B2 microglobin
Describe the antigen binding regions on immunoglobulin (Ig)
VH has 3 CDRs
VL has 3 CDRs