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After a transplant the immune system comes back in a fixed order, and the order explains most of what follows. Neutrophils in two to four weeks, natural killer cells within a month, B cells over several months to a year, and T cells last and slowest. Adults rebuild a narrower repertoire, because the thymus shrinks with age.
A review of immune reconstitution after allogeneic transplant states the structure plainly: reconstitution occurs in several phases, innate immunity being the first to regain function, and slow T cell reconstitution is regarded as primarily responsible for infections with latent viruses or fungi, for graft-versus-host disease and for relapse. It also notes that umbilical cord blood and haploidentical grafts were associated with prolonged immunodeficiency because of delayed reconstitution, which is the clearest illustration that the graft source sets the timetable.
Neutrophils and monocytes. Engraftment, conventionally the first of three consecutive days with a neutrophil count above 0.5 x 10^9/L, occurs at roughly two weeks after a peripheral blood stem cell graft and later after marrow or cord blood. Until then the person is neutropenic and is managed as such. Monocytes and dendritic cells recover over a similar period and become donor-derived.
Natural killer cells. The first lymphocyte population back, typically within the first month, and the reason the early post-engraftment period is not as defenceless as the T cell counts alone suggest.
B cells. Recovery over months. Naive B cells return first and the repertoire matures over a year or more; switched memory B cells, which carry serological memory, are the last and in some people never fully return. This is why antibody titres to vaccines given before transplant fall away, which is the subject of the revaccination record.
T cells, and the thymus. Two routes exist. Peripheral expansion of the mature T cells that came in the graft or survived conditioning is fast but produces a narrow repertoire skewed to memory phenotype, and in the first months after transplant this is essentially the only route: a recent review states that thymic egress of T cells is abrogated during the first three to six months after transplant, so early T cell reconstitution depends on peripheral expansion of engrafted donor T cells. Thymopoiesis, the production of genuinely new naive T cells with new receptor specificities, is the slow route, is measured by T cell receptor excision circles, and depends on a thymus that has not been destroyed by conditioning, age or GvHD.
Age is the dominant variable here and the measurement that established it is worth stating carefully. Douek and colleagues measured T cell receptor excision circles in people of different ages and showed that thymic output declines with age but that substantial output is maintained into late adulthood; it is not that the adult thymus does nothing, but that it does much less, more slowly, and from a smaller starting organ. Add conditioning toxicity to the thymic epithelium and, in allogeneic recipients, GvHD of the thymus itself, and the adult rebuilding a repertoire relies overwhelmingly on expanding what survived. The practical consequences are a repertoire that is narrower than it was even when the CD4 count looks adequate, slower recovery of responses to new antigens than to recalled ones, and a longer window of susceptibility to the latent viruses the review names.
Autologous transplant is different and quicker. There is no alloimmune reaction, no GvHD prophylaxis and no need for prolonged immunosuppression, so T cell and B cell recovery are faster, although serological memory is still lost and revaccination is still required.
After CD19 CAR-T the deficit is different again. The T cell compartment is not ablated, so general T cell immunity recovers from lymphodepletion within weeks to months; what is removed is the normal B cell lineage, on target, and with it the plasma cell precursors. That deficit can last years and is the subject of its own record.
Monitoring, where it is done, is by lymphocyte subset counts on flow cytometry, CD4 count, immunoglobulin levels and, in research settings, T cell receptor excision circles and repertoire sequencing. There is no randomised evidence that any intervention accelerates thymic recovery in adults, and the agents trialled for it, including thymic peptides, keratinocyte growth factor, growth hormone and sex steroid blockade, have not produced a treatment in routine use. That is the honest gap at the centre of this topic: the deficit is well described, well measured and currently not correctable.
Haematopoietic reconstitution proceeds from the stem cell outward in the order the lineages mature, so innate cells with short differentiation paths return first. Adaptive reconstitution has two sources with different speeds: homeostatic peripheral expansion, driven by interleukin-7 and interleukin-15 in a lymphopenic host, is fast but cannot create new receptor specificities; thymopoiesis can, but requires an intact thymic epithelial niche, which involutes with age and is damaged by conditioning and by GvHD.
Query for this technology: (TITLE:"Rebuilding an immune system: the timeline, lineage by lineage" OR ABSTRACT:"Rebuilding an immune system: the timeline, lineage by lineage" OR TITLE:"immune reconstitution after transplant" OR ABSTRACT:"immune reconstitution after transplant" OR TITLE:"T cell recovery after HSCT" OR ABSTRACT:"T cell recovery after HSCT" OR TITLE:"lymphocyte recovery after CAR-T" OR ABSTRACT:"lymphocyte recovery after CAR-T") AND (cancer OR tumor OR tumour OR oncology OR carcinoma OR lymphoma OR leukemia OR leukaemia OR myeloma OR sarcoma OR melanoma OR glioma). Results are unfiltered search hits about Rebuilding an immune system: the timeline, lineage by lineage, not a curated reading list.
Shares Preventing graft-versus-host disease, and what prevention costs, Infection risk after transplant and cell therapy, phase by phase, and the prophylaxis that follows it, Graft-versus-host disease (GVHD) and graft-versus-leukaemia, After a transplant or cell therapy: what to ask for and the tags rejuvenation, survivorship, transplant, immune.
Shares Preventing graft-versus-host disease, and what prevention costs, Cytopenias and myelosuppression, Graft-versus-host disease (GVHD) and graft-versus-leukaemia, After a transplant or cell therapy: what to ask for and the tags rejuvenation, survivorship, transplant.
Shares Cytopenias and myelosuppression, After a transplant or cell therapy: what to ask for, Autologous stem cell transplant (high-dose therapy), Allogeneic stem cell transplantation and the tags rejuvenation, survivorship, transplant.
Shares Blood counts that do not come back after CAR-T: ICAHT, After a transplant or cell therapy: what to ask for, Late effects and survivorship toxicity, Survivorship and late effects are neglected and the tags rejuvenation, survivorship, transplant.
Shares Graft-versus-host disease (GVHD) and graft-versus-leukaemia, After a transplant or cell therapy: what to ask for, Allogeneic stem cell transplantation, Late effects and survivorship toxicity and the tags rejuvenation, survivorship, transplant.
Shares Blood counts that do not come back after CAR-T: ICAHT, After a transplant or cell therapy: what to ask for, Late effects and survivorship toxicity, Survivorship and late effects are neglected and the tags rejuvenation, survivorship, transplant.
Shares Graft-versus-host disease (GVHD) and graft-versus-leukaemia, After a transplant or cell therapy: what to ask for, Allogeneic stem cell transplantation, Late effects and survivorship toxicity and the tags rejuvenation, survivorship, transplant.
Shares Revaccination after transplant: the schedules, and where the UK and the US differ, After a transplant or cell therapy: what to ask for, Autologous stem cell transplant (high-dose therapy), Allogeneic stem cell transplantation and the tags rejuvenation, survivorship, transplant.