When B and T cells multiple in the bone marrow, their offspring is slightly different.

The parent B cell will have a different antibody to its offspring B cells, and each of them a different antibody to their siblings.

In this way we increase the diversity of our immune system. For B cells, this maturation occurs in the bone marrow, while T cells mature in the Thymus.

If they survive the maturation process, B and T cells migrate to the lymph node and stay there until activated.

Most of the T and B cells, may never get used, however once a pathogen enters with the correct antigen marker to a T and B cell’s receptor they are activated.

Only the T and B cells which are activated undergo clonal selection. Unlike at the start, they divide into copies with identical receptors and antibodies.

However some of the copies will turn into memory cells, and remain in the lymph nodes.

This is so if the pathogen returns, they will respond much faster to it and you will barley feel it. Essentially giving you immunity to the pathogen.

Bellow is a diagram of Clonal selection in B cells