Test 2 Concepts Flashcards
Sensitization of Type 1 HS
Allergen breached tissue barrier
Phagocytosed by immature D cell
Travels to lymph node
Naive T cells recognise allergen derived pMHC on mature DC and become activated, differentiate into Th2 effector cells that help activate B cells. Activated B and Th2 cells leave the lymph node
Travel to where allergen entered body
Th2 cells produce cytokines that instruct plasma cells to produce IgE
IgE can bind directly to allergen or FCR on mast cells. Mast cells are sensitised for the effector stage
Excess IgE taken up by lymphatics
IgE encounters basophils in the blood and binds to FcRs to sensitise them
IgE encounters mast cells in other tissues and binds to FcRs to sensitise them
Effector stage of Type 1 HS (early stage)
Allergen enters tissue where sensitised mast cells are present
Allergen binds IgE molecules attached to mast cells via FcRs
Mast cells degranulate and secrete cytokines/chemokines
Breakdown of mast cells releases platelet-activating factor
Tissue-specfic symptoms of allergic response
Additional leukocyte recruited (eig eosinophils and sensitised basophils)
Type 1 HS
IgE medited, allergy atopy
<1-30min
Allergens cross-link IgE bound on mast cells and basophils induce degranulation
Asthma, hay fever, eczema, hives, food allergies
Type II HS
Direct antibody- mediated cytotoxic HS
Antibody (IgG or IgM)
5-8hr
IgM or IgM bind to cell-bound antigen, cell is destroyed by phagocytosis, complement activation or ADCC
Hemolytic anemias, Goodpasture’s syndrome
Antibody-mediated cytotoxic hypersensitivity
Autoantibodies against cell surface Ag
Complement-dependent cell lysis
C3b-opsonised phagocytosis
FcR-mediated antibody-dependent cell-mediated cytotoxicity, ADCC (NK cells and macrophages)
Type III hypersensitivity
Immune complex-mediated HS
Antibody (IgG or IgM)
4-6 hr
Immune complexes trigger complement activation; phagocyte FcR engagement leads to release of lytic mediators
Arthus reaction, aspects of rheumatoid arthritis and systemic lupus erythematosus
Immune complex-mediated hypersensitivity
Ab binds antigen in the blood (IC). Insoluble ICs form by cross-linking that lodge in small vessels. ICs enter tissues (kidneys, joints), drives local inflammation/tissue damage
How does pos and neg selection occur?
Positive selection occurs in the cortex when thymocytes are double positive for CD4 and CD8.
Thymic cortical epithelial cells mediate positive selection.
Developing T cells bearing antigen receptors that recognise self MHC with sufficient affinity survive.
T cells bearing TCRs that strongly recognise self peptide, self MHC are negatively selected.
DP cells express a successfully rearranged TCR with CD4 and CD8 on their surface
Positive selection rescues the T cell from default ‘death by neglect’
Those with high affinity interactions with dendritic cells presenting self MHC with self peptide undergo negative selection
Education of B cells
Educated to not recognise self antigens during their development,
Developing B cells with BCRs strongly recognize multivalent cell-surface self antigen undergo selection
B cell precursor rearranges its immunoglobulin genes
Immature B cell bound to self cell-surface antigen is removed from the repertoire
Mature B cell bound to foreign antigen is activated
Activated B cells give rise to plasma cells and memory cells
What happens when an APC presents self-peptide
DCs present self antigens derived from host cells, in the absence of pathogen attack
In the absence of pathogen attack the DC is not activated
DC encounters an autoreactive T cell that recognises the self MHC, self peptide combination it inactivates it
How do bacteria stick
Non-specific adhesion molecules (often reversible)
Specific adhesins - Gram-negative bacteria - Pili and Fimbriae. Outer membrane adhesins
Gram positive bacteria - cell wall proteins - Microbial surface components recognising adhesive matrix molecules
Enteropathogenic E.Coli attachment
Initial contact with intestinal cells mediated through bundle forming pilus (BFP)
Attaching - effacing lesion
- characterised by effacement of brush border microvilli and intimate attachment of bacteria to cell.
Mimicking - binding Ig
Staph - Protein A binds to antibody Fc region
Inhibits the activation of complement, phagocytosis and ADCC
Cell now covered in layer of host proteins = recognised as self
Mimicking - binding factor H
If C3b forms on a cell it can target it for complement-mediated damage.
Serum factor H binds to our cells and degrades any C3b forming on the surface to protect us from damage
H. pylori gastric coloniser
Attaches to gastric epithelium via Lewis b carbohydrate receptor with BabA adhesin
Chronic infection: Immune evasion methods
Evade host defense
LPS = poorly recognised by TLR4 -> low levels of cytokine production
Flagellum subunits = poorly recognised by TLR5 -> low levels of cytokine production
Vacuolating toxin A (VacA)
Inhibits phagosomal maturation. T/B cell proliferation, iNOS generation
Coating with plasminogen and cholesterol -> mimic host
