Antigen Recognition in Adaptive Immune System Flashcards
What do antigen receptors do for lymphocytes?
They serve roles in maturation of lymphocytes and all adptive responses.
Naive lymphocytes recognise antigens to initiate responses
Effector T cells and antibodies recognise antigens to perform their functions
What receptors do B and T cells express for antigens?
B cells (membrane bound antibodies)
T cells (TCRs)
What do lymphocytes do that allow them to distinguish many, closely related, chemical structures?
Antigen receptors are clonally distributed which means they each are specific for a distinct antigen and have unique receptors for different antigens.
What is the immune reportoire?
The entire collection of distinct clones
Are biochemical signals and cytokines antigen specific?
No, receptors are the same for cytokines.
How are B and T cell receptors similar?
Structurally they are similar. (The edge of the T cell receptor looks like one of the arms of an immunoglobulin)
Both T and B cells both use additional membrane proteins for signal transduction
What are the differences in recognition properties between B and T cells?
B cell receptors recognise much larger antigens directly without need for MHC. T cell receptors can only recognize MHC bound peptides.
BCRs recognise native conformation of antigens and for this reason have a much more broad specificity than TCRs.
BCRs can bind 2 antigens at the same time whereas TCRs only 1 antigen
How is the vast diversity of receptors structures in lymphocyte repertoire generated?
Many clones of lymphocytes with distinct specificity exist. (>10^9)
How are B and T cell receptors similar?
Structurally they are similar. (The edge of the T cell receptor looks like one of the arms of an immunoglobulin)
Both T and B cells both use additional membrane proteins for signal transduction . (the combined structures between the TCR/BCR and the other proteins form complexes.
Antigen/MHC binding results in clonal expansion response in both BCR and TCR responses.
Both BCRs and TCRs have polymorphisms with antigen-recognizing domains that are variable between clones. They also have constant regions required for structural integrity and effector functions which are conserved among all clones.
What is a complementarity determining region?
A region within each Variable region which forms part of receptor that binds to antigens.
Why is variation concentrated in small regions of receptor?
It maximises variability of antigen-binding while maintaining basic structure
What regions are variable in BCRs?
Heavy and light chains of membrane Ig.
What regions are variable in TCRs?
Variable regions of alpha and beta chains.
What are the 2 important functions of antigen receptors?
Specific antigen recognition by receptor and signal transduction mediated by different polypeptides.
What do receptor chains associate with for signal transduction?
Invariant membrane proteins
What do invariant membrane signalling proteins do?
Transmit signal to cytosol and nucleus which causes lymphocyte to divide, differentiate, or die.
What is a combined BCR/TCR with signalling molecule called?
BCR/TCR complex
What happens when antigen binds to antigen receptors on the molecular level?
Signalling proteins of receptor complex bundle together at the receptor and phosphorylation triggers complex signalling cascades activating transcription of genes and lymphocyte response.
What happens when antigen binds to antigen receptors on the molecular level?
Signalling proteins of receptor complex bundle together at the receptor and phosphorylation triggers complex signalling cascades activating transcription of genes and lymphocyte response.
What is an antibody molecule composed of?
4 polypeptide chains (2 heavy 2 light)
The heavy chains form the long middle chains and the light chains sit on top of the protein
How are the heavy chains attached to each other?
Disulfide bonds
How many variable (V) and constant domains do light and heavy chains have?
Light chain has 1 V and 1 C.
Heavy chain has 1 V and 3 - 4 C.
How are membrane bound immunoglobins (IgM) different to secreted immunogobins (IgG)?
Membrane bound IgM have 4 constant domains and a hydrophobic chain for membrane binding.
Secreted IgG antibodies have 3 constant chains
What do immunoglobin domains consist of?
2 layers of beta-pleated sheet held together by disulfide bridge.
How are adjacent Ig strands held together?
They are connected by short protruding loops
What regions of the variable domains are responsible for antigen recognition?
CDR (Complementarity Determining Regions) regions; CDR1, 2, and 3. 3 is the most variable.
What does the hinge region of antibodies do?
Allows the antigen to bend.
What are the types of light chains that can be present on antibodies?
2 types of light chains: κ and λ. Each B cell expresses one but never both.
What are the types of heavy chains on antibodies and how do they differ?
5 types: μ, δ, γ, ε, and α
They differ in C regions.
Does the type of light chain and heavy chain matter with the formation of antibody complexes?
No, each light chain can complex with any heavy chain
What does heavy chain divide antibodies into?
Isotypes/classes
What are the isotypes of antibodies?
IgA, IgD, IgE, IgG, IgM
What does the isotype say about the antibody?
Provides us with information about physical and biological properties.
Which classes of antibody get secreted as dimers?
IgA (form both monomers and dimers)
Which class of antibodies gets secreted as a pentamer?
IgM (they are usually membrane bound so this means they typically have a hydrophobic tail region meaning they get together prior to secretion due to the non-polar affinity)
Do IgD antibodies get secreted?
No, they are found on the membrane of naive B cells so they have low affinity (pointless secreting them)
What antibodies do naive B cells express?
IgM and IgD
{like MD students they are very naive to the world of medicine}
What stimulates B cell clones to expand?
Co-stimulation by antigen and helper T cells. They differentiate into antibody secreting progeny which can produce IgM and others which can produce other heavy-chain classes.