The Immune System

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5

The Role of the MHC

In infected cells, MHC molecules bind and transport antigen fragments to the cell surface, a process called antigen presentation.

A nearby T cell can then detect the antigen fragment displayed on the cell’s surface.

Depending on their source, peptide antigens are handled by different classes of MHC molecules.

Slide 29

Antigen Presentation by an MHC molecule

Antigen

Top view: binding surface

exposed to antigen receptors

Plasma

membrane of

infected cell

Antigen

Class I MHC

molecule

Slide 30

Class I MHC molecules are found on almost all nucleated cells of the body.

They display peptide antigens to cytotoxic T cells.

Class II MHC molecules are found on specialized cells: macrophages, B cells, and activated T cells…

Slide 31

Interaction of T cells with Antigen-Presenting Cells

Infected cell

Antigen

fragment

Class I MHC

molecule

T cell

receptor

(a)

Antigen

associates

with MHC

molecule

T cell

recognizes

combination

Cytotoxic T cell

(b)

Helper T cell

T cell

receptor

Class II MHC

molecule

Antigen

fragment

Antigen-

presenting

cell

Microbe

1

1

1

2

2

2

Slide 32

Class II MHC molecules are located mainly on dendritic cells, macrophages, and B cells.

Dendritic cells, macrophages, and B cells are antigen-presenting cells that display antigens on their surface to cytotoxic T cells and helper T cells.

Slide 33

Lymphocyte Development

The acquired immune system has three important properties:

Receptor Diversity

Lack of reactivity against host cells

Immunological Memory

Slide 34

Generation of Lymphocyte Diversity by Gene Rearrangement

Differences in the variable region account for specificity of antigen receptors.

The immunoglobulin (Ig) gene encodes one chain of the B cell receptor.

Many different chains can be produced from the same Ig chain gene by rearrangement of the DNA.

Rearranged DNA is transcribed and translated and the antigen receptor formed.

Slide 35

Origin of Self-Tolerance

Antigen receptors are generated by random rearrangement of DNA.

As lymphocytes mature in bone marrow or the thymus, they are tested for self-reactivity.