Jesus told his followers to beware of the leaven of the Pharisees and of Herod, but the disciples did not know what he meant by that. So, what could he have meant?
Well, since leaven, or yeast, inflates a dough, making a pound of dough rise to where it appears to be a pound and a half, the yeast of the Pharisees would be their excessive self esteem that makes them seem to be more important than they are.
St. Paul gives us a kindred passage in his First Letter to the Corinthians. In discussing the dispute between those who see either himself or Apollos the greater of the two. Paul asked, "What have you that you haven't received? Therefore, why should you boast as though you had not received?"
It is God, the giver of every human gift, who deserves credit for the gifts we have received.