Quaternary structure is the 4th level of protein structure. Proteins containing more than one polypeptide chain (subunits) form quaternary structure. The simplest form is a dimer, where 2 subunits interact. There are more complicated forms than this though. One of these is Haemoglobin, which carries oxygen in the blood, it has 4 subunits joined together by interactions. Small, reversible changes in the structure allow it to carry oxygen to our organs.