This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Transplantation, Subject name: Surgery
In order to activate helper/inducer T (CD41) lymphocytes, macrophages release 
Select and explain. 
A. Interleukin 1, B. Interleukin 2, C. Interleukin 3, D. Interleukin 4
A: Interleukin 1
Answer: A: Interleukin 1 
Explanation: 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.