T t t

Environmental signals (Ag, cytokines, chemokines)

Figure 1 Mechanisms of lymphocyte proliferation and homeostasis. In response to environmental signals, lymphocytes grow in size and enter the cell cycle. Most cells leave the cycle and die by apoptosis within a short time, however some cells survive for extended time as memory cells.

enter the cell cycle, the lymphocyte must grow to double the size of the resting cell before completing cell division. Thus, the cell activation and growth process is reversible while the cell cycle process is irreversible once a cell has reached the S phase of the cell cycle. Here the lymphocyte is committed to cell division and cannot return. In fact, the lymphocyte will die by apoptosis if cell cycle arrest is induced.

The period between sequential mitotic events is known as interphase. In adult humans, the average interphase survival of memory T cells has been estimated as 22 weeks while the survival of naive T cells has been estimated as 3.5 years. This is counterintuitive as the expectation had been that memory cells would be longer lived, thus supporting the notion that maintenance of immune memory requires frequent lymphocyte proliferation. The survival of any given lymphocyte clone, however, can be considerably longer, based on the product of the intermitotic lifespan and the number of population doublings. Estimates based on measurements of the in vitro population doubling capacity are that the lifespan of a given lymphocyte clone is in a range similar to the lifespan of a rodent or human. Lymphocyte interphase survival in mice is considerably shorter than that in humans.

Antigen-induced lymphocyte proliferation of mature cells in vivo does not normally occur in the blood and lymphatic ducts but, rather, in lymphoid tissues such as tonsil, lymph nodes, and spleen. It begins about 1-2 days following inoculation of antigen, and by 7 days lymph nodes may increase as much as 15 rimes normal size, reflecting a striking increase in lymphocyte size and number, as well as in lymphocyte ingress (homing). Several key aspects of lymphocyte proliferation and homeostasis are summarized in Table 1.

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