Complex sentence: MHC class two is formed of two chains, α and β, each having two domains--α1 and α2 and β1 and β2--each chain having a transmembrane domain, α2 and β2, respectively, anchoring the MHC class II molecule to the cell membrane.
Split into simpler sentences: MHC class two is formed of two chains, one α and one β, having two domains (α1 and α2, and β1 and β2). Each chain has a transmembrane domain anchoring the MHC class II molecule to the cell membrane.