Adaptive Immunity Flashcards
What is adaptive immunity?
Immunity that is induced by the presence of foreign material
Specific
Examples of adaptive immunity:
B cells
T cells
Where do B and T cells develop?
B cells: Bone marrow
T cells: Thymus
Where do B and T cells circulate in their inactive form?
Around their primary lymphoid tissue (site of development)
How are B and T cells activated?
By antigens in the secondary lymphoid tissue
E.g. lymph nodes
What happens when B cell encounters antigens (4)?
- Membrane-bound antibodies on the B cells bind to target antigen IgM within the B cell zone of lymph nodes
- Activated by antigen signals and ‘helper signals’
- B-cells clonally proliferate and become antibodies or a memory B cell
- IgM (high affinity) and IgG (produced by B cells responding to certain antigens) antibodies produced
Dendritic cells function
Act as a bridge between the innate and acquired immune system
Express antigens on their cell surface and present them to T cells
What is the complement system and how is it activated?
Activated in response to inflammation and creates a cascade of chemical reactions
MBL pathway (5):
- C3 is cleaved into the active C3a and C3b
- C3b can then cleave C5 into C5a and C5b
- C3b can then amplify the reaction via the alternative pathway, causing more C3 to cleave into C3a and C3b
- C5b produces the membrane attack complex, which inserts into cell walls and destroys the cell by letting salt and water in
- C3a and C5a are responsible for acute inflammation
What are acute phase proteins?
Proteins produced by the liver whose plasma concentrations increase or decrease in response to inflammation
Role of CRP
Activates complement via classical pathway
Rapidly increases during inflammation
Short half life - decreases rapidly after
Role of MBL:
Activates complement via MBL pathway
Role of CD4 + T cells
activate B cells & stimulate production of memory B cells
Role of CD8 + Killer T cells
Kill infected cells via perforin/granzymes/granulysin
Antibody structure:
Made up of 2 light chains and 2 heavy chains
Each has a unique variable region (antigen binding site) which is specific against one antigen
Where are membrane bound antibodies located?
On the surface of B cells
T-cell antigen receptor (TCR):
Can only recognise peptide antigens that are presented by MHC molecules
Class I MHC
Expressed on all nucleated cells
Present peptide antigens to CD8+ killer T cells
Class II MHC:
Expressed only on antigen presenting cells (e.g. dendrites, macrophages)
Present peptide antigens to CD4+ helper T cells
How do T cells encounter antigens (5)?
- Dendritic cells recognise and phagocytose antigenic debris
- In the presence of pro-inflammatory mediators, the dendritic cells mature and increase expression of stimulatory molecules on their surface
- Dendritic cells phagocytose the pathogenic antigens, break the antigens down into short peptides and load them onto MHC II molecules
- MHC II molecules transported to cell surface
- Co-stimulatory molecules enable T cell to respond to antigen and fully differentiate
Examples of ‘helper signals’
TH cells
PRR and PAMPS
Multiple antigens
Secreted antibodies:
Secreted by plasma cells
Present in serum and tissue fluid
IgM antibodies:
First antibody to be made in an inflection
Monomer when bound to B cell membrane but pentamer when released into plasma
IgM antibodies function:
B cell activation
Agglutination
Complement system activation through classical pathway
Aggluation:
When antibody cross-links multiple antigens producing clumps of antigens
Prevents viruses from binding to host cells
Enhances phagocytosis
IgG antibodies:
Most abundant antibody in plasma
Monomer
Dominant type during secondary response
IgG antibody function:
Foetal immunity
Complement activation
NK cell activation
Neutralisation
IgD antibodies and their function:
Second least abundant antibody
Monomers
B cell activation
IgA antibodies:
Second most abundant antibody
Monomer in serum
IgA functions:
Helps with neutralisation of pathogens
Dimmer in secretory fluids (e.g. colostrum in great milk)
Neonatal defence - protects GI tract of neonates
IgE antibody and function:
Least abundant
Monomer
Produced in allergic reactions
What two signals do B-cells require to become fully active?
Antigen signals
Helper signals
Germaline centre response
Production of memory B-cells
What do CD4 + T cells differentiate into?
T helper cells
What do T helper cells differentiate into?
Effector TH cells
Examples of effector TH cells:
TH1
TH2
TFH
Function of TH1 cells:
Migrates out of lymph nodes and travels to infected area
Secrete pro-inflammatory cytokines which stimulates production of ROS (macrophage-mediated)
What do CD8 + T cells differentiate into?
CTLs (Cytotoxic T Lymphocytes)
Role of IL-2
Provided from TH cells
Promotes differentiation and proliferation of antigen activated CD8+ cells into CTLs
How are antigens activated on CD8+ cells?
By MCH I
Role of CTLs (3):
Migrate out of secondary nodes to site of infection via transendothelial migration
Kill virally infected host cells by recognising pathogenic antigens attached to MHC class 1
Involved in killing cancer cells
Role of TH2 cells:
Secrete IL4, IL5 and IL6 which:
- Promote B cell proliferation
- Induces antibody production
Role of TFH cells:
Stimulated by presented antigen peptides + MHC II molecules on B cell
Stimulate the B cells to proliferate and differentiate (plasma + memory) by secreting IL4 and IL21
