Antigen processing Flashcards
What is an antigen?
Any molecule or molecular structure that cab be bound by a specific antibody or B - cell receptor
How does a cell “present” an antigen
Major histocompatibility (MHC) molecules Two main class of MHC molecules - class 1 and class 2 Third class - class ib Essentially hold up or present antigen for T -cell receptors
MHC complex genes
Located on chromsome 6 Similarity across mammals 3 classes Different allelic variants Arranged with other genes involved in antigen processing
Class 1 MHC
Teo polypeptide chains
Binds peptide of 8-14 amino acids with buried ends
alpha chain is encoded by a gene in the MHC region
Class 2 MHC
Two polypeptides chains alpha and beta chain both span the membrane ends of peptide binding groove are more open than in class I MHC so peptides are not buried
MHC class I and class II are expressed differentially
MHC class I molecules present peptides usually from intra -cellular compartments to CD8 cytotoxic T cells Therefore almost all cells express MHC class I to allow surveillance MHC class II molecules are ususally only found on B cells
Peptides from cytosolic pathogens for transport into the ER are generated in the cytosol (class I)
Proteins in the cell are continuously degraded, usually by the proteas one
Proteins can be tagged for degradation by ubiquitin
Proteins get broken down into short peptides in the cytosol
Peptides that bind MHC are actively transported into the ER
Protein molecules that are bound for the cell surface are translocated during synthesis into the lumen of the ER These proteins fold up and are assembled inside the ER Peptides are necessary for correct folding of MHC class I
MHC Class ib (HLA-E, HLA-F, HLA-G
Similar in structure to classical MHC class I molecules
Limited polymorphism
HLA-E and HLA-G play a role in reducing NK-cell mediated attack
MHC class ib (A-G)
Similar to MHC heavy chain
Limited polymorphism
Play a role in activating NK cells, T cells
B Cells process
1 binding to antigen
2 chemical signal
3 becomes plasma cell
4 releases antibodies
Structure of an antibody molecule
Roughly Y shaped
Constructured from paired heavy and light chains
Can be cleaved using papain at the hinges = Fab and Fc
Flexible at the hinges
Structure of IgG
2x heavy chains and 2x light chains
Linked by disulphide Honda
Interaction with antigen via variable region
Two type of light chain - lamda and kappa
Generation of lymphocyte antigen receptors
BCR and TCR are the means by which lymphocytes sense the presence of their antigens
Each cell near numerous copies of a clinal receptor
Wide variety of binding specificities is due to variation at the antigen binding site
Each chain not encoded by just one gene
Somatic hypermutation
Point mutations are introduced into the re arranged V regions of activated B cells which generates further diversity
