Complex sentence: A mitogen is a chemical, usually some form of a protein that encourages a cell to commence Mitosis, triggering plasma B cells can enter mitosis when they encounter an antigen matching their immunoglobulin.
Split into simpler sentences: A mitogen is a chemical, usually some form of a protein that encourages a cell to commence cell division, triggering mitosis. For example, plasma B cells can enter mitosis when they encounter an antigen matching their immunoglobulin.