Regeneration of B cell immunity

B cells reconstitution

B cells are virtually absent during the first 2 months after transplantation. The first B cells are naive while memory B cells appear only later. Hence, the reconstitution of the B cell compartment after transplantation resembles ontogeny. The same holds for the immunoglobulin isotypes produced. As in young children, levels of IgM, IgG1 and IgG3 normalize before IgG4 and IgA. The reconstitution of the B cell compartment is thus different from the reconstitution of the T cell compartment. The main reason for this is that B cells are produced at almost constant levels during life while the bulk of T cells are produced during the first years of life. After transplant, the daily production of B cells can begin as soon as the HSC start to produce enough B cell precursors. Obviously, the same is not true for T cells that need a functional thymus to mature. In normal adults, the thymus produces few T cells and in transplanted patients the thymic activity may considerably lower. Despite the production of enough naive B cells, the antibody response may be deficient. Because many antibody responses require T cell help, T cell dependent B cell responses cannot occur before the T cell compartment has been regenerated. Therefore, a functional recovery of B cell immunity generally takes 1-2 years.