Question

why can't cell multiple and regenerate tissue​

Answer 1

$\huge\mathfrak\red{☟︎︎︎ \: \: ᴀɴsᴡᴇʀ \: \: ✍︎}$

In humans, recent studies have shown that declines in thymic T-cell regenerative capacity begins relatively early in life, resulting in a limited capacity for T-cell regeneration by young adulthood.