Interleukin-2 (IL-2) is a small monomeric protein 133 amino acids in length. It is a very important cytokine in the immune response. It is secreted by T-lymphocytes and helps signal the body to produce more T-helper cells. It also controls B-cells, T-cytotoxic cells , natural killer cells and lymphokine activated killer cells.

The tertiary structure of protein is a four-helix bundle. It interacts with the IL-2 receptor, which sits on the cell surface of T-cells and transmits a signal into the cells which then promote proliferation.