Lymphocytes are types of White Blood Cells in the immune system. There are two main types of lymphocytes,
B lymphocytes,
T lymphocytes.
Plasma B lymphocytes produce large amounts of antibodies when you have a bacteria, virus or fungi.
Memory B lymphocytes remember things you've already had and change into Plasma B lymphocytes to then produce certain antibodies that can kill the bacteria before it turns into a disease.
The T lymphocytes are much more aggressive though,
things like cytotoxic T lymphocytes will destroy cells that are infected with a virus. Hope I helped. :)