IMMUN: Adaptive Immunity

Question 1

epitope

Answer 1

• part of the antigen that binds to the antibody

Question 2

how do B and T cells recognise pathogens

Answer 2

• B cells: have antibodies (4 chains) on surface - IgD and IgM
• T cells: T cell receptors (2 chains, a and B) and co-receptors (CD4/CD8)

Question 3

2 diff types of T cells

Answer 3

• T helper cell: has CD4 co-receptor, binds to MHC II molecules
• cytotoxic T cell: has CD8 co-receptor, binds to MHC I molecules

Question 4

what signals do Th cells need to be activated

Answer 4

• need 3 signals, only a dendritic cell can provide all 3
• 1) peptide:MHC
• 2) co-stimulation
• 3) cytokines

Question 5

how do dendritic cells activate T cells

Answer 5

• when patrolling body - good phagocytes, bad APCs
• if recognise pathogen - bad phagocytes, good APCs b/c upregulate costimulatory molecules (signal 2), then presents antigen on MHC II marker (signal 1)
• specific type of PAMP drives production of pro-inflammatory cytokines

Question 6

3 similarities between MHC I and MHC II markers

Answer 6

• transmembrane domains (cross plasma membrane)
• non-variable region: MHC I binds to CD8 (Tc) and MHC II binds to CD4 (Th)
• peptide-binding cleft (antigen binding site): many antigens can fit but only one can bind

Question 7

2 types of MHC markers

Answer 7

• MHC I (only a chain): on all nucleated cells b/c cytotoxic T cells need to be able to kill any virally infected cell, present endogenous material
• MHC II (a + B chain): subset of cells e.g. dendritic cells, macrophages, B cells etc, present exogenous material from engulfed pathogens

Question 8

MHC II pathway

Answer 8

• extracellular pathogen is recognised, engulfed and destroyed by phagocyte
• MHC II marker produced in the RER and joins with non-self antigen in a vesicle
• peptide/MHC II complex transported to cell surface and expressed
• recognised by CD4 Th cells

Question 9

MHC I pathway

Answer 9

• intracellular pathogen is moved to cytosol
• pathogen tagged w/ ubiquitin and cleaved in proteasome
• peptide moved into RER for loading onto MHC I markers (finds the one that can bind the strongest)
• peptide/MHC I complex transported to cell surface and expressed
• recognised by CD8 Tc cells

Question 10

3 characteristics of MHC markers

Answer 10

• co-dominant: MHC alleles from both parents are expressed > diversity in population
• polygenic: multiple genes contribute to building the MHC marker
• polymorphic: multiple alleles/variations of the MHC genes > diversity in the peptide binding cleft b/n diff ppl > allows any pathogen to be processed and presented

Question 11

antibody structure

Answer 11

• Y-shaped structure with 2 antigen binding sites
• mirror image
• variable (antigen binding site) and non-variable regions
• heavy and light chains
• Fab (fragment of antibody binding) and Fc (fragment crystallisation) regions
• 2 arms can hinge (Fab section)
• transmembrane region (crosses plasma membrane)

Question 12

lineage of T cells

Answer 12

Question 13

how do CD4 (Th) cells activate CD8 (Tc) cells?

Answer 13

• they also need 3 signals
• Th cells can provide signal 3 (cytokines)

Question 14

what do Tc (CD8) cells do?

Answer 14

• MHC I recognition pathway
• release perforin and granzymes to lyse infected and cancer cells
• production of antiviral and inflammatory cytokines
• once activated, only need signal 1 (antigen recognition) to kill

Question 15

how do B cells convert to Bm cells and plasma cells?

Answer 15

• ‘class switching’ - stimulated by IL-4
• IgM antibodies convert to other isotypes (irreversible)
• joining of variable, diversity and joining (VDJ) segments in diff combinations
• type of PAMP determines which isotype will be produced

Question 16

distribution of antibody isotypes

Answer 16

• IgG: blood, can cross placenta (second responder, high affinity)
• IgA (dimer): mucosa + breast milk
• IgD: signal B cells to be activated - BCR
• IgM (pentamer): blood and lymph - first responder - BCR, low affinity
• IgE: mast cells, connective tissue
• type and function determined by heavy chain

Question 17

functions of antibodies (PIAANOS)

Answer 17

• precipitation: makes pathogens insoluble, allowing them to precipitate out
• immobilisation
• agglutination
• activation of complement proteins and NK cells
• neutralisation: blocking of pathogen binding or neutralising toxins
• opsonisation: enhances phagocytosis and triggers complement proteins
• sensitisation of mast cells: IgE binds and creates cross links > degranulation

Question 18

2 characteristics of antigens

Answer 18

• immunogenicity: ability to stimulate lymphocytes to divide
• reactivity: react with antibodies and immune system to result in an immune response