T for Thymus!

After answering questions about my age, health and lifestyle for a covid-19 mortality calculator on this UK website, I was given a 1-in-20 risk of becoming infected and a 1-18 risk of dying if infected. It has become well-established that the risk of succumbing to a bout of covid rises dramatically with age, from essentially zero for the under-20s up to around 6% for reasonably healthy over-80-year-olds like me. The reasons for this disparity are not fully understood, but a decline in the effectiveness of the human immune system with advancing years is likely to be a significant factor. To give a readable account of the amazing complexity of this system is well beyond my knowledge and the scope of a short post, but I have managed to isolate one important component relevant to these statistics, namely the role of the T-cells

The Enemy

Pathogens are microbes that invade your body and make you ill. They include bacteria, viruses, pollen and fungi. A bacterium is a single cell organism that can independently reproduce in your body and may sometimes cause damage. In contrast, a virus, which  is much smaller, can only replicate by entering one of your cells and taking over a normal cell function to make copies of itself.

An antigen is a small part of a pathogen, typically a specific protein on its surface, that is used by your immune system to identify, track down and destroy the invading microbe. You could think of it as a banner advertising the pathogen’s presence to your immune system.

Your Defences

T-cells have many important functions in the human immune system:

• They kill your own infected cells that have been hijacked by a virus to replicate itself
• They activate other immune cells that directly attack the invading virus or other pathogen
• They regulate the overall immune response, and
• They store memories of earlier infections in order to mount a pre-emptive strike if one of them tries to re-infect you. 

T-cells begin life as a type of stem cell made in your bone marrow. From there they are carried in our blood stream to other organs, in particular to the thymus, a small gland located in the upper chest behind the sternum and in front of the heart. There they undergo a selection process which many don’t survive. Those that do then circulate round the body’s peripheral lymph-organs ready to be activated when they encounter a specific antigen. Once activated, the T cells will proliferate and differentiate into effector T-cells, which further diversify into T-cells with a variety of different functions; among these are cytotoxic T-cells whose main function is to kill your virally-infected cells but which can also kill tumorous cells and cells invaded by bacteria.

The thymus continues to grow after birth and reaches maximum size and activity around puberty. Thereafter it decreases in size and function and continues to atrophy until old age, eventually  turning to fat and becoming very hard to detect even under a microscope. Surely SARS-Cov2 exploits this decline in our once-vigilant defences.