Scientists discovered that when a dangerous fungus called Aspergillus terreus is grown together with immune cells, it produces a special compound called butyrolactone Ia that helps it hide from the body’s defense system. This compound works by reducing inflammatory signals that immune cells use to fight the fungus. The research shows that direct contact between the fungus and immune cells is needed to trigger this protective compound production, suggesting the fungus responds directly to the threat of immune attack.