Immune effector cells (e.g., lymphocytes, macrophages, dendritic cells, natural killer [NK] cells, and cytotoxic T lymphocytes [CTL]) work together to defend the body against cancer by targeting abnormal antigens expressed on the surface of tumors. Two major strategies are employed in cancer immunotherapy, so-called active immunotherapy and passive immunotherapy.

Active immunotherapy is intended to activate the immune system of the patient in order to mount a specific immune response able to recognize and destroy the tumor. Dendritic cells have a central role in stimulating immune responses by presenting antigens to immune effector cells. In some experimental immunotherapies, dendritic cells are either pulsed with tumor antigens (protein or peptides) or transfected with a viral vector expressing tumor antigen. If injected into a cancer patient, the modified dendritic cells could present tumor antigen in vivo, activating tumor-specific effector cells to mount an antitumor immune response.

Passive immunotherapy also uses dendritic cells, but instead of in vivo stimulation, immune effector cells (i.e., T lymphocytes) are activated in vitro and then transferred back into the patient where they recognize and destroy tumor cells. Autologous tumor-infiltrating lymphocytes (TILs), for example, have been successfully applied to treat patients with metastatic melanoma, by priming T cells extracted from the tumor milieu. TILs proliferate in vitro by exposure to high concentrations of IL-2, anti-CD3, and allo-reactive feeder cells.

Donated organs and tissues are often used to replace damaged tissue, but demand far outweighs the available supply. Stem cells, directed to differentiate into specific cell types, may provide a renewable source of replacement cells and tissues to treat a wide range of diseases, from Alzheimer's disease to rheumatoid arthritis.

Cell transplantation

Adult stem cell treatments have been successfully used for many years in bone marrow transplants to treat leukemia and related bone/blood cancers. Alternatively, umbilical cord blood stem cells can be used in hematopoietic stem cell transplants for patients lacking a matched donor.

Although tissue engineering is still in its infancy, this modern approach to cell transplantation has recently progressed as organs and tissues are now being grown in the lab from stem cells and transplanted into patients. In cell replacement therapy, stem cells are differentiated into cells of the target organ or tissue for later injection into the organ or tissue that needs repair. For instance, stem cells may be used in the future to replace damaged brain tissue in Alzheimer’s patients.

In some cases the transplanted cells must attach and replace damaged tissue, while in other cases the infused cells are only meant to release certain factors such as enzymes or growth factors. For example, in the case of insulin-secreting islets for the treatment of diabetes.