The Immune System

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B cells and T cells have receptor proteins that can bind to foreign molecules.

Each individual lymphocyte is specialized to recognize a specific type of molecule.

An antigen is any foreign molecule to which a lymphocyte responds.

A single B cell or T cell has about 100,000 identical antigen receptors.

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Antigen receptors on lymphocytes

Antigen-

binding

site

Antigen-

binding site

Antigen-

binding

site

Disulfide

bridge

Variable

regions

Constant

regions

Transmembrane

region

Plasma

membrane

Light

chain

Heavy chains

T cell

 chain

 chain

Disulfide bridge

Cytoplasm of T cell

T cell receptor

Cytoplasm of B cell

B cell receptor

B cell

V

V

C

C

V

V

C

C

C

C

V

V

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All antigen receptors on a single lymphocyte recognize the same epitope, or antigenic determinant, on an antigen.

B cells give rise to plasma cells, which secrete proteins called antibodies or immunoglobulins.

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Epitopes = antigen determinants

Antigen-binding sites

Antigen-

binding

sites

Epitopes

(antigenic

determinants)

Antigen

Antibody B

Antibody C

Antibody A

C

C

C

V

V

V

V

C

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The Antigen Receptors of B Cells and T Cells

B cell receptors bind to specific, intact antigens.

The B cell receptor consists of two identical heavy chains and two identical light chains.

The tips of the chains form a constant (C) region, and each chain contains a variable (V) region, so named because its amino acid sequence varies extensively from one B cell to another.

Secreted antibodies, or immunoglobulins, are structurally similar to B cell receptors but lack transmembrane regions that anchor receptors in the plasma membrane.

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Each T cell receptor consists of two different polypeptide chains. The tips of the chain form a variable (V) region; the rest is a constant (C) region.

T cells can bind to an antigen that is free or on the surface of a pathogen.

T cells bind to antigen fragments presented on a host cell. These antigen fragments are bound to cell-surface proteins called MHC molecules.

MHC molecules are so named because they are encoded by a family of genes (many unique / specific) called the Major Histocompatibility Complex.

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