Across a lifetime, the human body is not the fixed and faithful guardian we imagine it to be. Immunologists now remind us that allergies — long assumed to be the burden of childhood — can emerge at any age, as the immune system quietly revises its understanding of what is friend and what is foe. A protein tolerated for decades may one day be flagged as a threat, not because the world has changed, but because the body's inner logic has shifted. This recognition invites both humility and vigilance: knowing that our defenses can err is the first step toward navigating them wisely.