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. Mitogens trigger signal transduction pathways in which mitogen-activated protein kinase is involved, leading to mitosis.
Lectins as mitogens
Mitogens are substances capable of producing a non-specific, polyclonal activation of lymphocytes. Most of them are lectins or plant glycoproteins such as Phytohaemagglutinin (PHA), Concanavalin-A (Con-A) and Pokeweed Mitogen (PWM). From this kind of work, it appears that cross-linking of surface receptors is a crucial event in B-cell activation.
Urtica dioica contains a lectin which is a T cell mitogen distinguishable from classical T cell lectin mitogens by its ability to discriminate a particular population of CD4+ and CD8+ T cells as well as its capacity to induce an original pattern of T cell activation and cytokine production.