A(n) is an uncontrolled positive feedback loop between cytokines and leucocytes.

The given blank can be filled with cytokine storm.

Explanation:

Generally, influenza is self-restricting. Though serious cases can result in pneumonia and other issues, which can be deadly. However, such cases are very common in the elderly and in very young individuals. There are certain strains of the influenza virus, which are more dangerous to young adults in comparison to old or very young.  

The strains, which influence young adults are considered to comprise a cytokine storm, that is, a positive feedback loop, which produces in between the production of cytokines and leukocytes. This cytokine storm generates an acute inflammatory reaction, which results in brisk accumulation of fluid in the lungs, resulting in pulmonary failure. The very young or very old are less vulnerable to this influence as their immune systems are less strong.  

an active site is a region that is present on an enzyme where the reactants or the substrate molecules we can say bind and experience a chemical reaction. the substrate is basically a reactant whose concentration is changing and that is converting into a product after binding at the active site of an enzyme