He was referring to his literal body, soon to be beaten and bloodied. Bread without leaven was a fitting symbol of that body.
قَالَ يَسُوعُ وَهُوَ يُؤَسِّسُ عَشَاءَ ٱلرَّبِّ :‏ « هذَا يُمَثِّلُ جَسَدِي » .‏