Antigen recognition by lymphocytes I Flashcards
what are lymphocytes
A type of immune cell that is made in the bone marrow and is found in the blood and in lymph tissue
what are the two types of lymphocytes
B and T
what do B lymphocytes make
antibodies
what do T lymphocytes do
help kill tumor cells and help control immune responses
Coordination of immune response
T cell-independent antigens only produce IgM antibodies in small amounts
Vast majority of antibody responses require T cells to help
All T cells are specific for one antigenic determinant binding of antigen Clonal expansion
T cell receptor (TCR)
- Structure on T cell surface that recognises antigen
- Structurally related to immunoglobulin Fab region
- Recognizes antigen in form of a complex offoreign peptide bound to an MHC molecule
what is TCR similar to
Fab fragments of an antibody but not identical
what bond links alpha and beta chains
disulphide bridges
what do the majority of t cells have
αβ TCR
Major t cell classes
T cells fall into two major classes, which have different effector functions and are distinguished by the expression of the cell-surface proteins CD4 and CD8
on helper t cells TCR is associated with what
CD4 + CD3
on cytotoxic t cells TCR is associated with what
CD8+CD3
what is required to make an effective response to an antigen
CD4 and CD8
what do cd4 and cd8 mediate
additional contacts with MHC molecules
cd4 and tcr binding
when cd4 and TCR bind simultaneously to the same MHC class 11: peptide complex, the T cell is about 100x more sensitive to the antigen than if cd4 were absent
Summary I
Every lymphocyte is specific for a single antigenic structure
There are many variants (1011) of both B and T cells, each with a uniqueantigen specificity
The receptor for antigen on a B cell (BCR) is a form of the antibody it is programmed to make
The receptor for antigen on a T cell is the T cell receptor (TCR)
Lymphocytes are activated by binding antigen, which triggers them to divide to form a clone of antigen-specific cells
Activated B cells become plasma cells, secreting antibody specific for the antigen
While not identical, TCR is structurally related to immunoglobulin Fab
TCR recognises a combination of MHC plus processed antigen throughthe CDRs of its variable domains
summary 2
Helper T cells and cytotoxic T cells are major subsets of T cells
Helper T cells express CD3 & CD4; Cytotoxic T cells express CD3 & CD8
TCR on helper T cells recognises combination of antigenic peptide + MHCClass II on antigen presenting cells (APC)
CD4 also makes contact with MHC class II on antigen presenting cell
Adhesion molecules LFA-1 and ICAM-1 serve as accessory moleculesforming additional adhesive interactions between T cells and APC
Outcome is that APC delivers three kinds of signal to T cell – resulting inactivation, survival and differentiation
After binding antigen, CD4+ T cells secrete cytokines which coordinateimmune response, aiding B cells to make robust antibody response
