ABSTRACT
Qualitative and quantitative changes occur within the immune system during aging. Normal age-related T cell alterations include a decline in CD4+ cells, loss of naïve (antigenic virgin) cells, increase in memory (antigen experienced) cells, decline in T cell proliferative responses, and narrowing of the T cell receptor repertoire. T cell age-associated alterations are secondary to increased extrathymic T cell reconstitution following post puberty thymic involution combined with peripheral antigen driven T cell maturation and expansion. Age-related B cell changes are predominately qualitative and include loss of high affinity antibodies, increase in low affinity and autoantibodies, impaired isotype switching, and hindered antibody responses to vaccination. B cell related changes appear secondary to age-related impairment of T cell function. Immune reconstitution after hematopoietic stem cell transplantation (HSCT), whether from autologous or allogeneic stem cells, initially arises by means of extrathymic reconstitution. It is, therefore, at onset dominated by changes similar to those of an aged immune system. After HSCT, it may take 1 to 2 or more years for thymic reconstitution of a T and B cell phenotype and function normal for the recipient’s age. The younger the recipient, the more rapid thymic reconstitution occurs. The health of the graft as well as patient survival appears to correlate with the rapidity of shift from extrathymic to thymic T cell reconstitution following HSCT.
