IgM is produced first in a response to foreign antigen, but its concentration declines rapidly. With five Y-shaped monomer subunits forming a pentamer structure, IgM is very effective in binding many copies of the same antigen and agglutinating them, but is too big to cross the placenta.
IgG is the most abundant class of antibodies in circulating blood, a monomer capable of passing through vessel walls to protect cells and tissues. In some species, including humans, it crosses the placenta to pass on the mother's immune protection to the fetus. Produced after IgM in an immune response, it is much more effective against bacteria, viruses, and toxins.
IgA is secreted as a dimer (two subunits) into milk, sweat, saliva, and tears. It is especially impor-
tant in colostrum, the secretion before milk production begins that is the only way some newborn animals receive their mother's antibodies. IgAprevents bacteria and viruses from binding to epithelial cell surfaces, especially in the digestive tract.
IgE antibodies bind to the surfaces of mast cells and basophils with the arms of the Y-shaped monomer extended. Foreign antigens bind to the ends of the Y arms and trigger these cells to release hista-mine and other chemicals that cause the inflammation of allergy. IgE is also the antibody that attacks parasites inside the body, such as worms.
IgD molecules are monomers located mainly on the surfaces of B cells, apparently acting as receptors for the antigen that is recognized by each B cell and triggers its activation.
mice, and hamsters. Laboratory mice have been highly inbred into strains where all the animals are genetically identical and their genes and antigens are well known. Studies on these mice have been essential in determining how the immune system normally works, and how it fails to work in autoimmune diseases and the inability to prevent cancer cells from proliferating.
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