Human Anatomy, Physiology, and Medicine. Anything human!
In a nutshell, T cells (or their progenitors, rather) first divide actively and start to differentiate in the bone marrow, then migrate to thymus where they continue to mature (including T cell receptor assembly, psoitive and negative selection, double CD4+/CD8+ phase etc.) and then move to the periphery, where the last maturation events take place. In the thymus and the periphery the cells do not divide actively, but when they have reached the so-called naive stage (CD45RA+) they divide if they encounter a specific peptide:MHC for their T cell receptor.
Unless the TCR signal is very strong (may cause anergy or apoptosis) or very weak (causes no activation) the cells multiply manyfold and quickly adapt the CD45RO+ phenotype, which indicates that they have become memory cells (central or effector), which is basically the final step of the T cell maturation cycle.
The whole process is very complex, however, and it includes several other changes in the cell phenotype and markers as well. So bear in mind that the above text is just a brief summary.
The spleen is more than just a storage space for immune cells. It is an active immunological organ and the immune cells within it function somewhat similarly to lymph nodes. Although a human can survive without the spleen, persons who have undergone splenectomy are more prone to certain infections, including dangerous septic infections by capsulated bacteria such as Streptococcus pneumoniae, Meningococcus pneumoniae, or Haemophilus influenzae.
However, the previous poster asked what happens in the spleen, not thymus. The spleen has the so-called periarteriolar lymphoid sheaths that consist of T cells. In addition to T cells, dendritic cells migrate there to present antigens to the T cells. That is what happens in the spleen. And the thymus has already been covered in several previous posts in this thread. Kindly read some of the earlier answers before you post a duplicate one.
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