If a person eats an olive-sized portion of matzah on Pesach, he fulfills the mitzvah. If he eats an olive-sized portion of chametz (leaven), he is liable to kareis (spiritual excision). If chametz of one of these species gets mixed up with other types, it violates the prohibition against possession over Pesach. If a person vows to abstain from bread and sustenance, only the five grains (listed in 1:1) are prohibited to him; this is the opinion of Rabbi Meir. The Sages say that if a person vows to abstain from grain, only these five species are prohibited to him. These five grains are obligated in both challah and tithes.
The following are obligated in challah but exempt from tithes: the portions reserved for the needy in the field, something declared ownerless, first tithes whose terumah has been separated, second tithe and sanctified produce that have not been redeemed, what’s left from the omer, and produce that has not reached one-third of its final size. Rabbi Eliezer says that produce that has not reached one-third of its final size is exempt from challah.