Nature's Bounty® Immune 24 Hour + with Ester-C®, the only Vitamin C with 24 hour immune support. Every dose of Immune 24 Hour+ provides advanced, long-lasting immune support to help keep you healthy. Clinically studied Ester-C is the only Vitamin C that stays in your white blood cells for up to 24 hours and lasts up to 2x longer than regular Vitamin C. Highly bioavailable Ester-C® can increase Vitamin C levels in the body, which is important for white blood cell function, as white blood cells play a vital role in the immune system and contribute to the health of the upper respiratory tract.