Lymphocytes Flashcards
Why do we need immune memory?
To fight infections on subsequent infection
Why do we need adaptive immunity?
Gives memory and specificity to clear infection
Improves efficacy of innate immune response
Focuses a response in the sire of infection and the organism responsible
What are the main features of adaptive immunity?
Has memory
Needs time to develop
What is immune memory?
Consequence of clonal selection
Antigen-specific lymphocytes are the cellular basis
Characterised by more rapid and heightened immune reaction
Can confer life-long immunity
Basis for vaccine
What are the two types of adaptive immune response?
Humoral (B-cell) Cell mediated (T-cells)
Define antigen
Molecules that act induce an adaptive immune response
Define epitope
The region if an antigen which the receptor binds to
What do T-cells recognise?
Linear epitopes in the context of MHC (Major histocompatibility complex)
e.i A chain of amino acids
What to antibodies recognised?
Structural epitopes
What is clonal selection?
Each lymphocyte bears a single unique receptor
Interaction between a foreign molecule and that receptor leads to activation of adaptive immune response
Differentiated effector cells of the lineage will bear the same receptor
Self-specific receptors are deleted
What are the problems with antigen diversity?
Exposed to large no. of different microbes
Immune system must be able to respond to all
How do we deal with antigen diversity?
Encode a massive repertoire
10^10 different antibody molecules can be encoded
How is the enormous diversity generated?
Functional genes for antigen receptors do not exist until they are generated during lymphocyte development
Each BCR chain is encoded by separate multigenerational families on different chromosomes
Single gene that can recombine in several ways
“Immunoglobulin gene rearrangement”
What are the two types of T-cell?
T-helper cells
Cytotoxic T cells
What are the main features of the T-cell receptor?
Part of a complex of proteins on the cell surface
Variable region made by gene reassortment
Recognises antigen fragments presented by other cells
What is the role of the Major histocompatibility complex?
Central role in defining self and non-self
Encoded by HLA genes in humans
Presents antigens to T-cells
Critical in surgery and donor matching
What are the main features of Major histocompatibility complex gene expression?
Polygenic
Co-dominant
What is MHC class I?
All nucleated cells, although at various levels, levels may be altered during infection or by cytokines
What is MHC class II?
Normally only on ‘professional’ antigen presenting cells
May be regulated by cytokines
How does MHC I acquire antigen?
Constantly presenting proteins made within the cell on the cell surface
Always shows what proteins are being made e.g. self/non-self/viral
How does MHC II acquire antigen?
Specific antigen presenting cell e.g. dendritic
Circulate looking for infectious material
Engulf infectious material and present on MHC II molecule
What T-cells do MHC I interact with?
CD8- Killer
What T-cells do MHC II interact with?
CD4- Helper
What to CD8 cells do?
Kill their targets by apoptosis
Fragmentation of nuclear DNA
How does MHC recognition work?
Recognises non-self and kills it
What do CD4 cells do?
Produce cytokines
Cytokines have diverse actions on a wide range of cells
Influence the outcomes of the immune response
What do B-cells do?
Make antibodies
Recognise soluble antigens
Need help from other sources to produce antibody
What are antibodies?
Very specific, functional molecule that recognises proteins/ pathogens?
What are the 5 classes of antibody?
IgG IgA IgE IgD IgM
What are the three core roles of antibodies?
Neutralisation
Opsonisation
Complement activation
Where do B-cells come from??
Derived from stem cells in the bone marrow
Migrate into the circulation
Mature B-cells are specific for a particular antigen
Specificity resides in the BCR
What are the main features of the BCR?
Surface bound antibody- encodes the antibody the cell will make
Have unique binding site, before the cell ever encounters the antigen
Thousands of identical BCR on the surface
What help do B-cells need in antibody production?
Require an accessory signal from:
Microbial constituents (only IgM)
Helper T-cells
How do thymus-independent antigens work?
Directly activate B-cells without T-cells
Often bacterial/polysaccharide (need to be repetitive structure)
The second signal is provided by the microbial constituent or by an accessory cell
Is not very effective
How does the Thymus-dependent approach work?
Membrane bound BCR recognises the antigen
Receptor bounds antigen is internalised and degraded into peptides
Peptides associate with ‘self-molecules’ and is expressed at the cell surface
Complex recognised by matched CD4 T-helper cell
B-cell is activated
