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Glossary of HIV/AIDS Terms

for Health Care Providers

Glossary of HIV/AIDS Terms


Without a known cause.

Idiopathic thrombocytopenia

Low level of platelets without a cause.


Intravenous (injection) drug use.

Immune response

The activity of the immune system against foreign substances.

Immune system

The body's complicated natural defense against disruption caused by invading foreign agents (eg, bacteria, viruses). Acquired, or learned, immune response occurs when specialized cells (dendritic cells and macrophages) process pieces of infectious agents. The ultimate result is the creation of antibody-producing B cells and cytotoxic T lymphocytes.

Immune thrombocytopenic purpura (ITP)

Also called idiopathic immune thrombocytopenic purpura. A condition in which the body produces antibodies against the platelets in the blood, which are cells responsible for blood clotting. ITP is very common in persons with HIV.


A natural or acquired resistance to a specific disease. Immunity may be partial or complete, long lasting or temporary.


Protection against an infectious disease by vaccination, usually with a weakened (attenuated) or killed form of the disease-causing microorganism. While people are usually immunized against an infectious disease by getting vaccinated, having a disease such as measles, mumps, or rubella one time usually prevents or "immunizes" a person from getting this disease again.


Capable of developing an immune response; possessing a normal immune system.


Refers to an immune system in which the ability to resist or fight infections and tumors is subnormal.


Breakdown of the immune system in which certain parts of the immune system no longer function. This condition makes a person more susceptible to certain diseases.


(Antigen) A substance capable of provoking an immune response.


The ability of an antigen or vaccine to stimulate an immune response.

Immunoglobulin (Ig)

Also called immune serum globulin. A class of proteins also known as antibodies made by the B cells of the immune system in response to a specific antigen. There are five classes of immunoglobulins.

Immunoglobulin A (IgA)

A class of antibodies that is secreted into bodily fluids such as saliva. IgA protects the body's mucosal surfaces from infections.

Immunoglobulin D (IgD)

A class of antibodies that is present in low concentration in serum.

Immunoglobulin E (IgE)

A class of antibodies involved in antiparasitic immunity and in allergies.

Immunoglobulin G (IgG)

The dominant class of antibodies and the one that provides long-term protection against infection. During pregnancy, IgG crosses the placenta to the fetus and protects it against infection. Also called gammaglobulin.

Immunoglobulin M (IgM)

A class of antibodies that is made by the body as the initial response to an infection or immunization. If IgM is made in response to an immunization, a booster shot will result in a "switch" from IgM to mostly immunoglobulin G.


Any substance that influences the immune system.


Any agent or substance that triggers or enhances the body's defense; also called immunopotentiator.


A state of the body in which the immune system is damaged and does not perform its normal functions. Immunosuppression may be induced by drugs (e.g., in chemotherapy) or result from certain disease processes, such as HIV infection.


Treatment aimed at restoring an impaired immune system.


A plant or animal toxin (ie, poison) that is attached to an antibody and used to destroy a specific target cell.

In utero

Before birth; literally, in the uterus.

In vitro

An artificial environment created outside a living organism (eg, a test tube or culture plate) used in experimental research to study a disease or process.

In vivo

("In life") Within living organisms. (Often refers to animal or human studies.)


The number of new cases (e.g., of a disease) occurring in a given population over a certain period of time. Also called seroincidence.

Inclusion/Exclusion criteria

The medical or social standards determining whether a person may or may not be allowed to enter a clinical trial. For example, some trials may not allow persons with chronic liver disease or with certain drug allergies; others may exclude men or women, or include only persons with a lowered T-cell count.

Incubation period

The time interval between the initial infection with an infectious agent (e.g., HIV) and the appearance of the first symptom or sign of disease.

Indeterminate test result

A laboratory test result that does not provide a clear answer. Either additional laboratory studies should be performed or the test should be repeated.


The state or condition in which the body (or part of the body) is invaded by an infectious agent (eg, a bacterium, fungus, or virus) that multiplies and produces an injurious effect (active infection). As related to HIV, infection typically begins when HIV encounters a CD4+ cell.


An infection capable of being transmitted by direct or intimate contact (e.g., sex).

Informed consent

A process during which the patient learns the key facts about HIV testing — including what will occur during HIV testing and counseling and the purpose and benefits of HIV testing — before deciding whether to allow testing to proceed.


The process of administering fluid, other than blood, to an individual by slowly injecting a solution of the compound into a vein. Infusions are often used when the drug is too toxic or the volume is too large to be given by quick injection.


The introduction of a substance (inoculum; e.g., a vaccine, serum, or virus) into the body to produce or increase immunity to the disease or condition associated with the substance.

See Vaccine.

Institutional review board (IRB)

A committee of physicians, statisticians, researchers, community advocates, and others that ensures that a clinical trial is ethical and that the rights of study participants are protected.

Insulin resistance

A condition in which the body is unable to respond to and use the insulin it produces. As a result, the pancreas secretes more insulin into the bloodstream in an effort to reduce blood glucose levels.


An enzyme that inserts HIV's genes into a cell's normal DNA. It plays a vital role in the HIV infection process.

Integrase inhibitors

A class of anti-HIV drugs that prevents the HIV integrase enzyme from inserting viral DNA into a host cell's normal DNA.

Intent to treat

Analysis of clinical trial results that includes all data from patients in the groups to which they were randomized (ie, assigned through random distribution) even if they never received the treatment.


See Drug-drug interaction.


One of a number of antiviral proteins that control the immune response. Interferon (IFN) alpha is secreted by a virally infected cell and strengthens the defenses of nearby uninfected cells. A manufactured version of IFN alpha is an FDA-approved treatment for certain conditions. Interferon gamma is synthesized by immune system cells (natural killer cells and CD4 cells). It activates macrophages and helps promote cellular immunity.


Substances (cytokines) that are released from immune and other cells. There are many types referred to as interleukin (IL) 1, 2, 3, 4, 5, etc.


Relating to or situated in the small, narrow spaces between tissues or parts of an organ.


An action or strategy to change a particular problem or outcome or accomplish a specific result (e.g., use of ART to prevent HIV disease progression).

Intramuscular (IM)

Injected directly into a muscle.


Time during labor and delivery.

Intravenous (IV)

Of, or pertaining to, the inside of a vein. An injection made directly into a vein.

Intravenous immunoglobulin (IVIG)

A sterile solution of concentrated antibodies extracted from healthy people. IVIG may be used to prevent certain infections in persons with low or abnormal antibody production. IVIG is injected into a vein.


An individual part of an organism (such as a spore, bacteria or virus) that has been separated (as from diseased tissue, contaminated water, or the air) from the whole.

Isoniazid (INH)

An orally administered drug used to eliminate tuberculosis infection in people without active disease. INH is also administered in combination with other drugs to treat active tuberculosis