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B cells are immune cells responsible for the production of antibodies, antigen presentation and cytokine secretion. They are released as immature cells by the bone marrow, and their total number, as well as the composition of the B cell population circulating in peripheral blood, dynamically changes during the course of life.
Immediately after birth total B cell count increases 2-fold. This increase is due to an active production and release of B cells by the bone marrow, that reaches a maximum between 0 and 12 months of age. Then B cell count remains high until 2 years old, when a gradual decrease starts, leading to approximately a 6.5-fold decrease in adulthood. Starting from 2 months of life, memory B cells increase too. Their number remains high until the age of 5 years old, when it starts decreasing to adultlike values. In adulthood, total B cell number remains relatively stable until aging.
However, aging is associated with the reduction of B cell production by the bone marrow. Recent analyses suggest that this age-related decline is due to the increased production of inflammatory cytokines that can inhibit their synthesis, such as IL- 1, IL-6 and TNF-α. In young people, short-term inflammation may stimulate the production of myeloid cells, such as monocytes, that help to promptly respond to infections; however, the chronic inflammation associated with aging may translate into a continuous inhibition of B cell production. Moreover, memory B cells and new plasma cells (that is, the more advanced differentiation step of mature B cells, able to secrete large amounts of antibodies) gradually decrease after 60 years of age.