This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Physiology
IL-2 is secreted by 
Select and explain. 
A. CD4 lymphocytes, B. CD8 cells, C. Macrophages, D. Neutrophils
A: CD4 lymphocytes
Answer: A: CD4 lymphocytes 
Explanation: IL-2 is a typical four a helix cytokine and is produced primarily by activated CD4+ T cells, although expression by naive CD8+ T cells, dendritic cells, and thymic cells has also been repoed (8, 9, 10, 11). In T cells, IL-2 synthesis is tightly regulated at the mRNA level by signals from the TCR and CD28 (12).