A β sheet is one of the fundamental structural building blocks of proteins, consisting of adjacent, extended strands of amino acids (β strands) that are bonded together by interactions between backbone amide and carbonyl groups. β Sheets can be parallel, in which case the adjacent strands run in the same direction, or antiparallel, where adjacent strands run in opposite directions. All immunoglobulin domains are made up of antiparallel β-sheet structures. A β barrel or a β sandwich is another way of describing the structure of the immunoglobulin domain.