Chimeric antigen receptors (CARs) are unique receptors designed to target specific tumor antigens for functional reprogramming of T lymphocytes. Since T lymphocytes are genetically engineered to express these artificial receptors to target tumor cells, this therapy may be referred to as immunotherapy, gene therapy, or oncotherapy. The human defense system can effectively recognize self and nonself molecules, including bacteria, viruses, and abnormal tumor cells. Tumor cells were identified based on their antigenicity and immunogenicity acquired through the expression of foreign antigens. However, tumor cells have the potential to disrupt the immune system to exert their advantage, resulting in insufficient anti-tumor immunity and tumor survival and progression. Immunotherapy is also called biologic therapy because the body's immune system is naturally able to detect pathogens and tumor cells. In recent years, immunotherapy has become an important branch of the treatment of similar diseases. However, the mechanism of protection may be different. Some immunotherapies boost the immune system, while others target tumor cells directly. Each treatment type has advantages and disadvantages, depending on the disease type. Tisagenlecleucel (Kymriah) is a product used to treat acute lymphoblastic leukemia (ALL) in patients younger than 25 years of age. Similarly, axicabtagene ciloleucel (Yescarta) is approved for patients with large B-cell lymphomas, such as non-Hodgkin's lymphoma (NHL), and those whose cancers are refractory and relapsed or have not responded to other therapies. With an improved understanding of the immune system, a number of innovative immunotherapies are being developed using approaches that include inducing the immune system to act in a specific way to target malignant cells. Another approach involves the delivery of immune components, such as synthetic, modified immune proteins that have been genetically engineered to target tumor antigens.

CAR T cell therapy can be defined as a treatment in which a patient's T cells are genetically modified in the laboratory to kill tumor cells. The mechanism of adoptive CAR T-cell therapy involves collection of the patient's blood, isolation of T cells from a peripheral-blood sample by means of leukapheresis, transduction of the cells with a vector encoding the CAR gene, ex vivo expansion of the CAR T cells, and infusion into the patient to target the tumor.

An increasing number of CAR T-cell therapies are being developed and tested in clinical studies. Although there are important differences between each therapy that may influence the ways in which they act in the patient's body, they all have similar components. Each CAR connects to the cell membrane. Some receptors are extracellular and some are intracellular. The CAR moiety that extends from the cell surface usually consists of antibody fragments or domains made in the laboratory. Which domains are used affects the extent to which the receptor recognizes or binds tumor cell antigens. The interior of each CAR has signal and "costimulus" domains. After the receptors interact with the antigen, they transmit signals into the cell. The different domains used can affect the overall function of the cell.

Investigators have also begun to rethink the source of immune cells for CAR T-cell therapy - using T cells that have been harvested not from patients but from healthy donors, with the goal that "off-the-shelf" CAR T-cell therapies can be used immediately, rather than having to be manufactured for each patient.

All FDA-approved CAR T-cell therapies rely on viral delivery of genetic material into T cells to produce a CAR. But for off-the-shelf CAR T cells, which are now being tested in clinical trials, gene-editing technologies such as TALON and CRISPR are being used to induce CAR production from donated T cells.

Other off-the-shelf CARs use different types of immune cells, such as natural killer (NK) cells. Many of these researches are still in early stages, but some CAR NK cell therapies have been tested in clinical trials.

It is important to reconsider not only the source of T cells and the type of immune cells, but also where treatment is actually administered. For example, several groups are using nanotechnology and mRNA-based approaches that allow the creation of CAR T cells in vivo.