In cell culture experiments, purified B cells expressing IgM can be induced to switch to producing IgE by stimulating them with an antibody to CD40 (a stimulatory antibody) plus the cytokine IL-4. In an individual undergoing an immune response, these signals would normally be provided by:

TFH cells in the germinal center

Explanation:

The Tfh cells, present in the germinal center, are responsible for maturing and stimulating the class change of immunoglobulins (such as the change from IgM to IgE). This change could stimulate the new classes of immunoglobulins to act with stimulating antibodies to produce an immune response.

It is important to point out that the germinal center is the activated point of a lymphoid follicle that contains B and T lymphocytes, which act in the memory of the cells of the immune system.

multiply it by 5 in order for you to understand i cant give an exact answer.