Immune deficiency		A breakdown or inability of certain parts of the immune system to function, thus making people susceptible to diseases that they would not ordinarily develop.
Immune response		The body's reaction to foreign microbes and other antigens. This response may neutralize or eliminate the microbes and provide protective immunity against future encounters with microbes or toxins.
Immunity		Natural infection- or vaccine-induced resistance to a specific disease or toxin. Immunity may be partial or complete, specific or nonspecific, long-term or transient.
Immunization		The process of inducing immunity by administering a vaccine, thereby "teaching" the immune system to recognize certain microbial antigens and thus prevent infection or illness when it subsequently encounters the infectious agent.
Immunogenicity		The extent to which an immunogen or vaccine stimulates immune responses.
Immunotherapy		Treatment of, or prophylaxis against, disease by attempting to produce active or passive immunity after infection has occurred.
Incidence		The rate of occurrence in a defined time period of new cases of a particular disease in a population being studied.
Investigational new drug application (IND)		A request for authorization from the FDA (Food and Drug Administration) to administer an investigational drug or biological product to humans. Such authorization must be secured to allow clinical trials in the United States with an investigational vaccine or drug for indications that have not been approved by the FDA.
Induration		An area on the skin at the site of a tuberculin skin test (PPD) injection that is hard and raised, with defined edges. (See PPD).
Infiltrate		An area of visible density in the lung fields seen on a chest X-ray in a person with pulmonary TB disease or another pulmonary infection/pneumonia. It is produced by an abnormal collection of fluid and cells at the site of Mycobacterium tuberculosis infection in the lung.
Informed consent		A written agreement signed by volunteers participating in clinical research, indicating their understanding of: (1) why the research is being done; (2) what researchers hope to learn; (3) what will be done during the trial, and for how long; (4) what risks are involved; (5) what benefits, if any, can be expected from the trial; (6) what other interventions are available; and (7) the participant's right to leave the trial at any time.
Intradermal (also intracutaneous)		Within or between the layers of the skin. Some vaccines are administered intradermally, whereas others use a percutaneous (multiple puncture), subcutaneous or intramuscular injection method. A TB study of BCG in 2006 comparing intradermal and percutaneous administration showed no difference in efficacy.
In-vitro test		A laboratory test done in an environment other than a living body, for example in a test tube or culture vessel, and used to study microbes, diseases and biological processes.
In-vivo test		A test done in a living body, such as an experimental animal or human subject.
Isoniazid (INH)		A drug used in combination with other drugs to treat TB disease or used by itself for treatment of latent TB infection. Although it is relatively safe, it may cause hepatitis and other adverse reactions in some patients.
IRB (institutional review board; also research in human subjects review committee)		A group of physicians, medical ethicists, statisticians, non-scientist community representatives and others. The board or committee reviews and must approve clinical research protocols before they can be initiated. IRBs ensure that a trial is ethical and that the rights and safety of the participants are adequately protected.