The humoral system

  • Antibodies- Immunoglobulin, a tetramer -two identical "light" and "heavy" chains.
  • Variable region, includes the N-terminal end of the heavy and light chains, and is responsible for antigen recognition.
  • Constant region- C-terminal end of the heavy and light chains are the same for each class of antibody-light chains, and five major classes of heavy chain giving IgM, IgG, IgA, IgD, and IgE.

Variable and Constant, an example using the alphabet instead of DNA bases.

  • antibody 1. ABCDEFGLMNOP
  • antibody 2. ABCDEFGVWXYZ
  • variable= antigen binding, the right end of each "antibody"
  • constant= removal of antigen, the left end and the same sequence for each antibody class. Ie. IgM all have the same constant sequence, but have different variable sequences.

The Antibody Genes-Dr. Susumu Tonegawa , Nobel Prize .

DNA from a myeloma cell (secretes antibody) and embryonic cells were digested with restriction endonucleases and the DNA fragements separated by electrophoresis. The DNA fragments were transfered to membrane and hybridized with a radioactive probe complementary to the constant or the variable parts of the antibody. The "C" and "V" domains are on separate fragments of DNA from embryonic cells, but are on a new single fragment of DNA from the myeloma. This result showed that antibody genes are formed by DNA rearrangement.

Antibody Genes

  • The chromosome includes many coding regions, called "V" for variable, "D" for diversity, and "J" for joining. These are passed from generation to generation by the germ cells.
  • Stem cell -> B cell, DNA segments for both heavy (VDJ) and light (VJ) chains are randomly combined.
  • Variation -number of gene fragments and the joining process.
  • Clonal expansion and selection - If an antigen appears that binds to one of the ramdomly generated membrane antibodies of a B-cell, then that clone can be selected to proliferate and differentiate into memory and antibody producing plasma cells.

The antibody gene rearrangements occur prior to exposure to antigen

Somatic mutation- antibody improves with time

  • During clonal selection, secondary immune responses produce much better antibody.
  • Mechanism involves specific mutations in the variable antibody gene coding for antibody. New memory cells have random mutations.
  • Selection by antibody binding insures that the highest affinity antibody produced will be selected.

Class switch

- DNA rearrangement changing the heavy chain constant gene in memory cells.

Hybridomas

-Fuse antibody producing cells with myelomas. Clones are immortal and produce monoclonal antibody. A tool for Biology and Medicine

Move on to Regulation of the immune response

wjgrimes@u.arizona.edu