In order to activate helper/inducer T (CD41) lymphocytes, macrophages release

Interleukin 1 (IL-1) is a thymocyte mitogen produced by activated macrophages as well as many other types of cells (e.g., monocytes, dendritic cells, Langerhans cells, neutrophils, microglial cells). It induces interleukin 2 production by the helper T cell, which initiates a cascade of immunoregulatory and inflammatory functions.