Info

C molecules*

1 r

Antigen-binding site

Made up of three CDRs in VH and three CDRs in VL domains

Made up of three CDRs in Va and three CDRs in Vp domains

Peptide-binding cleft made of a1 and a2 (class I) and a1 and pi (class II) domains

Nature of antigen that may be bound

Macromolecules (proteins, lipids, polysaccharides) and small chemicals

Peptide-MHC complexes

Peptides

Nature of antigenic determinants recognized

Linear and conformational determinants of various macromolecules and chemicals

Linear determinants of peptides; only 2 or 3 amino acid residues of a peptide bound to an MHC molecule

Linear determinants of peptides; only some amino acid residues of a peptide

Affinity of antigen binding

Kd 10-7-10-11 M; average affinity of Igs increases during immune response

Kd 10-5-10-7 M

Kd 10-6-10-9 M; extremely stable binding

On-rate and off-rate

Rapid on-rate, variable off-rate

Slow on-rate, slow off-rate

Slow on-rate, very slow off-rate

*The structures and functions of MHC and TCR molecules are discussed in Chapters 6 and 7, respectively.

CDR, complementarity-determining region; Kd, dissociation constant; MHC, major histocompatibility complex; (only class II molecules depicted); VH, variable domain of heavy chain Ig; VL, variable domain of light chain Ig.

to an antigen, B cells differentiate into plasma cells that secrete antibodies. Secreted forms of antibodies accumulate in the plasma (the fluid portion of the blood), in mucosal secretions, and in the interstitial fluid of tissues.

When blood or plasma forms a clot, antibodies remain in the residual fluid called serum. Serum lacks coagulation factors but otherwise contains all the proteins found in plasma. Any serum sample that contains detectable antibody molecules that bind to a particular antigen is commonly called an antiserum. The study of antibodies and their reactions with antigens is therefore classically called serology. The concentration of antibody molecules in serum specific for a particular antigen is often estimated by determining how many serial dilutions of the serum can be made before binding can no longer be observed; sera with a high concentration of antibody molecules specific for a particular antigen are said to have a high titer.

A healthy 70-kg adult human produces about 2 to 3 g of antibodies every day. Almost two thirds of this is an antibody called IgA, which is produced by activated B cells and plasma cells in the walls of the gastrointestinal and respiratory tracts and is actively transported across epithelial cells into the lumens of these tracts. The large amount of IgA produced reflects the large surface areas of these organs.

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