Imagine having a spare copy of your immune system on ice, ready to replace your existing one should you fall victim to AIDS, an autoimmune disease, or have to undergo extensive chemotherapy for cancer.

An Anglo-American company called Lifeforce has received permission from the US Food and Drug Administration to do just that. The firm collects 480-millilitre samples of blood from healthy individuals, extracts the white blood cells and stores them as an insurance policy against future disease. The service comes at a price, though: around $800 for taking the initial sample then $25 per month for storing the cells …