Q.B-lymphocytes are produced from :

The human thymus grows until about puberty (~12 years) and then involutes.

The thymus is where T-lymphocytes mature. It is relatively large and active in infants and children, continues to grow until puberty (around 12 years of age), reaches its maximum size there, and thereafter gradually decreases in size and is largely replaced by fat in adults.

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B-lymphocytes are produced and mature in the bone marrow.

Both B- and T-lymphocytes arise from common lymphoid progenitor stem cells in the bone marrow, but while T-cells migrate to and mature in the thymus, B-cells complete their maturation in the bone marrow itself (the 'B' historically referring to the Bursa of Fabricius in birds, the mammalian equivalent being the bone marrow). Mature B-cells then circulate to secondary lymphoid organs …

The thymus functions most actively during early life.

As the primary lymphoid organ responsible for the maturation and selection of T-lymphocytes, the thymus is relatively large and highly active in infants and children, when the immune repertoire is being established. After puberty it begins to shrink (involute) and is progressively replaced by fatty tissue, so its hormonal and lymphopoietic activity declines markedly in adulthoo …