Immunology Chapter

5 – Spring 2020

Read chapter 5. Below are some notes to help you organize the material.

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What are the two major types of T cells?

· CD4 T cells and CD8 T cells will be the main two types of T cells we will discuss. The following section will go into more detail about these two types of T cells, but for now know that (Fig. 5.12)…

· CD4 T cells (aka helper T cells) have the co-receptor CD4 on their surface and recognize antigen bound to MHC class II.

· CD8 T cells (aka killer T cells) have the co-receptor CD8 on their surface and recognize antigen bout to MHC class I.

How do T cells recognize antigen?

· Via a T-cell receptor (TCR)

· A highly variable antigen-specific receptor

· Recognize mainly protein antigens bound to a human glycoprotein (aka MHC)

· Have similarities and differences to Immunoglobulin.

· Fig. 5.1 shows the structural difference between Ig and a TCR

· The T cell receptor consist of two separate chains (alpha and beta or gamma and delta  (Fig. 5.7)

· There are actually two types of T cells based on TCR. We will focus primarily on α: β T cells because they are the primary T cells in circulation for most vertebrates.

· A few points to know about γ:δ T-cells:

1. They are more abundant in tissues compared to circulation

2. Behave differently from α: β T cells in that they can have effector functions

3. Antigen recognition is not dependent on antigen presentation in MHC

· Each chain has four basic parts

1. A cytoplasmic tail

2. A transmembrane region

3. A constant region

4. A Variable region

· A genomic recombination event (similar to that of somatic recombination in B cells) allows for variation in antigenic binding sites of T cells. (Fig. 5.3)

· You are not responsible for knowing this process in detail, but definitely know that it occurs.

· T-cell receptors form a T cell receptor complex with CD3 proteins (Fig. 5.6)

· A T cell receptor can recognize antigen but alone it is not going to be able to transduce a signal when antigen is bound.

· T-cell receptors can have major functions when activated by antigen (Fig. 5.13).

· Kill sick cells. (CD8 T cells)

· Send cytokine signals to activate macrophages. (CD4 T cells)

· Send cytokine signals to activate differentiation and further cell division of activated B cells. (CD4 T cells)

What are MHC molecules?

 

· MHC stands for major histocompatibility complex.

· Present peptide (protein) antigens to T cells.

· Antigen is processed within a cell, bound to MHC and then the MHC with antigen is inserted into the cytoplasmic membrane. Only then can a T cell recognize antigen within a MHC molecule. Refer to Fig. 5.10

· There are two types of MHC.

· Refer to Fig. 5.11:  MHC class I and MHC class II

· MHC (HLA) class I

· Expressed by a broad range of cell types (Fig. 5.25)

· Presents antigen from intracellular pathogens (Fig. 5.27) for example viruses.

· Binds short fragments of peptides from the intracellular pathogens (8-10 amino acids in length).

· Exhibits promiscuous biding specificity

· Can bind thousands of peptides with different amino acids sequences

· Presents antigen to CD8 T-cells (Fig. 5.15)

· MHC (HLA) class II

· Expressed by a limited number of cell types (Fig. 5.25)

· Presents antigen from extracellular pathogens (Fig. 5.27). ie. most bacteria

· Binds large fragments of peptide (usually 13 to 25 amino acids in length)

· Exhibits promiscuous biding specificity

· Presents antigen to CD4 T-cells (Fig. 5.15)

 

· MHC molecules are encoded within the major histocompatibility complex

· The major histocompatibility complex

 

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