<P> In immunology, MHC tetramers can be used in tetramer assays, to quantify numbers of antigen - specific T cells (especially CD8+ T cells). MHC tetramers are based on recombinant class I molecules that, through the action of bacterial BirA, have been biotinylated . These molecules are folded with the peptide of interest and β2M and tetramerized by a fluorescently labeled streptavidin . (Streptavidin binds to four biotins per molecule .) This tetramer reagent will specifically label T cells that express T cell receptors that are specific for a given peptide - MHC complex . For example, a Kb / FAPGNYPAL tetramer will specifically bind to Sendai virus specific cytotoxic T cell in a C57BL / 6 mouse . Antigen specific responses can be measured as CD8+, tetramer+ T cells as a fraction of all CD8+ lymphocytes . </P> <P> The reason for using a tetramer, as opposed to a single labeled MHC class I molecule is that the tetrahedral tetramers can bind to three TCRs at once, allowing specific binding in spite of the low (10 - 6 molar) affinity of the typical class I - peptide - TCR interaction . MHC class II tetramers can also be made although these are more difficult to work with practically . </P>

Haemoglobin evolved as a tetrameric protein from monomeric globin