The thymus is a strange organ. It sits in the chest between the lungs and consists of two fleshy lobes, just a few centimetres wide. It is bigger in children than in adults and changes from a grayish-pink to yellow colour as we age. The scientific consensus right up to the 1960s was that it didn’t serve any significant function, other than appearing to be a graveyard for what we now call T-lymphocytes.
Indeed, in 1963 the great British biologist Sir Peter Medawar even doubted this graveyard theory. “We shall come to regard the presence of lymphocytes in the thymus as an evolutionary accident of no very great significance.” In short, the thymus was regarded as a vestigial organ, something left behind by evolution.
That all changed with Jacques Miller’s experiments on mice. It was the late 1950s and Miller had begun his PhD research on viral leukaemia at the Chester Beatty Laboratories in London, now part of the Institute of Cancer Research. He soon switched his interest to the role of the thymus. “The kind of leukaemia I was looking at begins in the thymus. So I thought I would study the effects of thymetcomy [thymus removal] at birth,” Miller later recalled.
His research subjects were thymectomised mice, which he observed started to lose weight and became sick once they were weaned. Skin grafting showed they were immunodeficient because Miller could end up with mice with four of five different skin colours on their back without any signs of tissue rejection. “Obviously the thymus had something to do with the immune system,” he said.
In a seminal 1961 paper published in The Lancet, Miller proposed that the thymus was an essential player in the immune system. Although he was met with much scepticism from the likes of Medawar, Miller was eventually vindicated by further studies proving that it was the place were a certain class of white blood cells were produced. They became known as T-lymphocytes, with the “T” standing for Thymus.