Q.Old people have weak immune system. What could be the reason?

The thymus is one of those organs you probably never think about — and for most of your life, it barely exists. But early on, it plays a role that nothing else can replace.

Start with the intuition. Imagine a factory that produces raw, untrained soldiers. They have the right uniform and the right weapons, but they don't know friend from foe. If you sent them into battle like that, they'd attack your own troops. So before deployment, they must go through a rigorous training camp where they learn to recognise the enemy and ignore their own side. The thymus is that training camp — specifically for a type of white blood cell called a T-lymphocyte (or T-cell). The 'T' stands for thymus.

Now the precise biology. The thymus is a soft, bilobed gland located in the upper chest, just behind the breastbone and in front of the heart. It is most active in children and shrinks with age, replaced by fatty tissue. That's why young children are more vulnerable to infections — their immune system is still being 'trained' — but also why they recover from many illnesses faster: the training camp is still running at full capacity. …