Immunotherapy or biological therapy is the treatment of disease by activating or suppressing the immune system. Immunotherapies designed to elicit or amplify an immune response are classified as activation immunotherapies, while immunotherapies that reduce or suppress are classified as suppression immunotherapies. Immunotherapy is under preliminary research for its potential to treat various forms of cancer.

Cell-based immunotherapies are effective for some cancers. Immune effector cells such as lymphocytes, macrophages, dendritic cells, natural killer cells, and cytotoxic T lymphocytes work together to defend the body against cancer by targeting abnormal antigens expressed on the surface of tumor cells. Vaccine-induced immunity to COVID-19 relies mostly on an immunomodulatory T cell response.

Therapies such as granulocyte colony-stimulating factor (G-CSF), interferons, imiquimod and cellular membrane fractions from bacteria are licensed for medical use. Others including IL-2, IL-7, IL-12, various chemokines, synthetic cytosine phosphate-guanosine (CpG) oligodeoxynucleotides and glucans are involved in clinical and preclinical studies.