Antigen Processing & Presentationaka MHC Flashcards
Innate vs Adaptive immunity
Innnate
The immune system you are born with
Does not change during life
Fixed number of preformed pathogen recognition receptors
Advantage:
Fast
Many cells
Disadvantage:
may not be able to react to something new
Non specific
No memory cells
Adaptive
Extremely variable receptor set (Specific)
Extreme ability for clonal expansion
B cells adapt to completely new structures
Advantages:
Can react to anything
Immunological memory
Disadvantage:
Slow
Two types of adaptive immune cells
B lymphocytes- produce antibody
T lymphocytes –
1. Cytotoxic T cells: Kill infected cells (CD8+) (Similar to the NK cells) (Cytotoxic cells)
2. Helper T cells: Co-ordinate the immune response and help other immune cells (CD4+)
(Activate the B cells), (Produce Cytokines)
What is MHC?
MHC molecules are cell surface proteins that present peptides from pathogens to T cells, essential for T cell activation and regulation.
Differentiate between MHC Class I and MHC Class II in terms of structure, function, and the type of antigens they present.
MHC Class II (Extracellular): Present on APCs (dendritic cells, macrophages, B cells).
Structure- α1, α2 chains and β1 and β2 chains
Has 2 transmembrane domains
Displays exogenous peptides
Interact CD4 Helper T cells
MHC Class I (Intracellular): Present on all nucleated cells.
Structure- a1, a2, a3 and B2 macroglobulin chain
Has 1 transmembrane domain
Displays endogenous peptides
Interact with CD8 Cytotoxic cells
Antigen-Presenting Cells (APCs)
Name three antigen-presenting cells and describe their roles in activating T cells.
Dendritic Cells: Activated by PAMPs
Capture antigens, migrate to lymph nodes, and present them to T cells.
When the dendritic cell presents a specific antigen in the lymph node to the T cell or the B cell that lymphocyte which has a receptor which is specific to the antigen that gets activated, it enters clonal expansion.
Macrophages: Phagocytose pathogens and present antigens to helper T cells.
B Cells: Internalize antigens through their B cell receptors and present them to helper T cells.
How does MHC polymorphism contribute to the immune response at the individual and population levels?
MHC polymorphism allows individuals to present a wide variety of peptides, enhancing the immune response’s adaptability. At the population level, it increases the likelihood that some individuals will effectively respond to diverse pathogens.
Antigen Processing for MHC Class I and II
MHC Class II: The cells are activated and extracellular antigens are taken up by endocytosis, degraded in lysosomes, and loaded onto MHC II in endosomes. MHC II-peptide complexes are then transported to the cell surface to present to CD4+ T cells.
Invariant chain- It protects peptide binding cleft of MHC II and prevents binding of nonspecific peptides.
Slide 28
MHC Class I: Intracellular antigens are degraded by the proteasome. Peptides are transported into the ER by TAP, loaded onto MHC I, they travel through the golgi body out of the cell and presented on the cell surface for recognition by CD8+ T cells.
MHC class I assembly:
1.MHC class I heavy chain is stabilized by calnexin until B2-microglobulin binds
2.Calnexin is released. The peptide-loading complex forms from TAP and helper proteins, MHC class I heavy chain, and B2m
3. A peptide delivered by TAP
binds to the class I heavy chain, forming the mature MHC class I molecule
4. MHC class I dissociates from the peptide-loading complex and is exported from the endoplasmic reticulum
What is cross-presentation, and why is it important in the immune response?
Cross-presentation allows APCs, especially dendritic cells, to present extracellular antigens on MHC I molecules, crucial for activating CD8+ T cells against pathogens that do not directly infect APCs.
