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- They are receptors on the cell membrane of T cells
- Have an alpha & beta chain
- ANCHORED on surface of cell membrane
- Very specific ( in terms of structure ; must bind to peptide antigen & MHC )

T Helper Cell TCR
- Antigen binding site on the variable region of the TCR
- Co-receptor, CD4 present. This is just a marker protein

T Cytotoxic Cell TCR
- Antigen binding site on the variable region of the TCR
- Co-receptor, CD8 present. This is just a marker protein

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♡ MHC has 2 types - 1 and 2
♡ MHC 1 works with the cytotoxic T cells, MHC 2 works with the helper T cells

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Gene cluster, whose proteins present antigens to T cells

MHC Class 1
* Found on Most cells in the body
* Binds to CD8 molecules on T cells
* 1 Anchor to Cell Membrane, this is known as trans-membrane domain

MHC Class 2
* Found on Macrophages, Dendritic cells & B cells
* Binds to CD4 molecules on T cells
* 2 Anchor to Cell Membrane , this is known as trans-membrane domain

Polygenic ;  Multiple copies of the same gene one one chromosome
- Individual possess a set of MHC molecules, with different ranges of peptide-binding specificity

Polymorphism ; Many allele of the same gene in a population
MHC IS BOTH POLYMORPHIC & POLYGENIC

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-  BCR has heavy and light chains. It consists of a Fab and Fc region.
- BCR can bind to both soluble or cell associated antigens.

- TCR has alpha and beta chains, without an Fc region

- TCR will only bind to peptide antigens with either a Class I or Class II MHC (major histocompatibility complex), or continuous epitopes on peptide antigens.

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T cell receptor (TCR) is found on T cells.  T cells BIND ONLY to processed peptide fragment presented on MHC on APC.

MHC Class 2 is found on Antigen-presenting cells, which are Macrophages, Dendritic cells & B cells. They are responsible for presenting exogenous proteins.  Peptide antigen is presented by MHC class 2 molecule on surface of target cell to the T cell receptor of CD84 T cells.

MHC Class 1 is then found on all nucleated T cells. They are responsible for presenting endogenous proteins.  Peptide antigen is presneted by MHC class 1 molecule on surface of target cell to the CD8 T cells.

NO RECOGNITION :
- Unbound antigen
- Wrong peptide presented
- Wrong MHC Class

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Human MHC is called human leukocyte antigen (HLA) that is unique to each person
Class I MHC: HLA-A, HLA-B, HLA-C
Class II MHC: HLA-DP, HLA-DQ, HLA-DR

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1. Recognition
- Viral antigen on MHC Class 1 binds to TCR of Cytotoxci T cells
2. Activation
-  Cytokines released
3. Effector Function
-  Perforin releases granzymes
- Perforin pore is formed on the target cell
- Granzymes enter the perforin pore
- Apoptosis of infected cell

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Through PRR, PAMP activates dendritic cells that can then activate naive CD4 and CD8 T cells
3 signals provided by dendritic cells:

1. Peptide antigens presented on MHC

2. Co-stimulation signal (B7/CD80)

3. Cytokines

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Phagosome - contains a phagocytosed particle enclosed within a part of the cell membrane

Lysosome - contain digestive enzymes to digest excess or worn out organelles, food particles, and engulfed viruses or bacteria

Endosome - sorting and delivery of internalized material from the cell surface and the transport of materials from the Golgi to the lysosome or vacuole

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Cross-presentation is the ability of certain antigen-presenting cells to take up, process and present extracellular antigens with MHC class I molecules to CD8 T cells

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Positive and Negative Selection of T cells

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Positive and Negative Selection of T cells

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Positive:
- Cells in the thymus present peptides on their own MHC class I and class II molecules which allow immature T cells to bind
- T cells that do not bind undergo apoptosis
- T cell receives survival signals and is positively selected IF a T cell’s TCR successfully binds to the MHC complexes on the thymic cells
-

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TC or CD8 cells are activtated & kills infected cells (or tumour cells) through secretion of perforin and granzymes or activation of the death receptor.

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Helper T cells are important in adaptive immunity, as they are required for almost all adaptive immune responses. Helper T cells help activate B cells to secrete antibodies and macrophages to destroy ingested microbes,, depending on their subtypes. Cytotoxic T cells to kill infected target cells