Abscess: A contained collection of pus buried in tissues, organs, or confined spaces.

Adenosine (Ado): A compound that helps make up RNA, a substance important in the formation of proteins and transmission of genetic information.

Adenosine deaminase (ADA): An enzyme found in healthy people that breaks down deoxydenosine (dAdo) and adenosine (Ado) in the body. The absence of ADA results in a build-up of these metabolites that are toxic to the immune system, leaving patients vulnerable to infections.

Allogeneic: Having cell types that are antigenically (genetically) distinct. In transplantation biology, denoting individuals (or tissues) that are genetically similar but not identical.

Amniocentesis: A procedure performed to obtain a sample of the fluid surrounding a developing fetus. Cells harnessed from this fluid are used to diagnose genetic disorders.

Antibiotics: A chemical substance with the ability to kill or inhibit growth of microorganisms.

Antigens: Any substance capable, under appropriate conditions, of causing an immune response. These include toxins and foreign proteins, as well as bacteria and tissue cells.

Autosomal recessive/autosomal recessive disease: Genetic term meaning 2 copies of a gene must be inherited in order to be affected (for the disease or trait to develop).

Candida: A genus of parasitic fungi that resemble yeasts. Occur especially in the mouth, vagina, and intestinal tract, where they are usually benign but can cause illness. 

Catalyze: To increase the rate of a chemical reaction.

Cell-mediated: Relating to or being the part of immunity or the immune response that is mediated primarily by T cells rather than by antibodies secreted by B cells.

Chemotherapy: The use of chemical agents in the treatment or control of disease or mental disorder.

Clearance: A quantitative measure of the rate at which a substance is removed from the blood.

Chronic: Persisting over a long period of time.

Clone: One of a group of genetically identical (barring mutation) cells or organisms derived from a single common ancestor.

dAXP: Deoxyribonucleotides, a nucleotide that contains deoxyribose and makes up DNA.

Deficiency: Ashortage of some substance necessary to health.

Deoxyadenosine (dAdo): One of the 4 principal nucleosides of DNA. The enzyme adenosine deaminase (ADA) converts this toxic molecule into nontoxic deoxyinosine.

Diagnosis: The determination of the nature of a disease, and distinguishing one disease from another.

Engraft: The process by which transplanted stem cells become a permanent part of the immune system of a patient receiving a bone marrow transplant or gene therapy.

Enzyme: Complex proteins that are produced by living cells and catalyze specific biochemical reactions at specific body temperatures.

Erythema: Abnormal redness of the skin due to congestion of the small blood vessels (as in inflammation).

Erythrocyte: Red blood cells. Assist in the transport of oxygen.

Experimental: Of, relating to, or based on experience or experiment. Experimental treatments are not approved by the FDA.

Failure to thrive: Failure to grow and gain weight as expected.

Flow cytometry: A technique for identifying and sorting cells and their components (such as DNA) by staining with a fluorescent dye and detecting the fluorescence usually by laser beam illumination.

Growth factor receptor: A protein molecule to which a signaling molecule (such as a cytokine or a hormone) may bind or attach.

Hematopoietic: Of, relating to, or involved in the formation of blood cells.

Hemolytic anemia: Anemia caused by excessive destruction of red blood cells.

Humoral: Relating to or being the part of immunity or the immune response that involves antibodies secreted by B cells and circulating in bodily fluids.

Immunity: A condition of being able to resist a particular disease.

Immunocompromised: The impairment or weakening of the immune response by administration of immunosuppressive drugs, irradiation, malnutrition, or by some disease processes (eg, cancer or the acquired immune deficiency syndrome).

Immunodeficiency: Inability to produce a normal amount of antibodies or immunologically sensitized T cells especially in response to specific antigens.

Immunogenicity: Producing an immune response.

In utero: In the uterus; before birth.

Intramuscular injection: An injection into the muscle.

Lymph nodes: Part of the lymphatic system. Collections of tissue composed of honeycombs filled with lymphocytes that collect and destroy bacteria and viruses. When the body is fighting an infection, lymphocytes multiply rapidly and produce a swelling of the lymph nodes.

Lymphocyte: A type of immune cell located in the lymph nodes, originating from stem cells.

Lymphopenia: Reduction in the number of lymphocytes (T, B, and NK cells) in the blood. Also known as lymphocytopenia.

Meningitis: A disease that may be either a mild illness caused by any of numerous viruses (as various coxsackieviruses) or a more severe usually life-threatening illness caused by a bacterium (especially the meningococcus or the serotype designated B of Hemophilus influenzae), that may be associated with fever, headache, vomiting, malaise, and stiff neck, and that if untreated in bacterial forms may progress to confusion, stupor, convulsions, coma, and death.

Metabolism: The chemical changes in living cells by which energy is provided.

Metabolite: Any substance produced by metabolism or by a metabolic process.

Missense: Pertaining to a mutation which changes the unit of genetic code so that a different amino acid is made.

Mitogen: A substance that induces mitosis (ie, cell division); DNA, RNA, and protein synthesis; and proliferation of lymphocytes.

Molecular: Of, pertaining to, or composed of molecules. More than 12 molecular (or genetic) causes of SCID have been discovered.

Mutation: A relatively permanent change in hereditary material.

Nervous system: The bodily system that is made up of the brain and spinal cord, nerves, ganglia, and parts of the receptor organs.

NK cells: Natural killer cells are capable of mediating cytotoxic reactions without prior sensitization against the target.

Opportunistic: Description of a microorganism that does not ordinarily cause disease but can under certain circumstances.

Opportunistic infections: Usually harmless but can cause illness when the host's resistance is impaired.

Orphan drug: The term for a drug that has a limited commercial value because the disease that it treats is so rare.

Pegylated: A process by which the molecule polyethylene glycol (PEG) is attached to another chemical in order to create a therapeutic drug. Pegylation helps certain drugs to remain in circulation in the body long enough to have value as a medical therapy. The chemicals might otherwise be rapidly eliminated by the kidneys before they can have the desired effect.

Phenotype: An observable characteristic or trait of an organism.

Platelet: Tiny, disk-shaped cellular fragments that play a role in blood clotting.

Pneumonia: A disease of the lungs that is characterized especially by inflammation and consolidation of lung tissue, is accompanied by fever, chills, cough, and difficulty in breathing, and is caused chiefly by infection.

Prenatal: Occurring, existing, performed, or used before birth.

Proliferate: To grow by rapid production of new parts, cells, buds, or offspring.

Recessive: A gene that produces an effect in offspring only when it is passed on by both parents.

Recombinase: An enzyme that accelerates the exchange of short pieces of DNA between two long DNA strands, creating new combinations of genes.

Retroviral vector: A modified retrovirus (virus with a genome made of RNA) used as a vector to transport and manipulate DNA. Used in gene therapy.

SAHase: S-adenosylhomocysteine hydrolase, the enzyme that breaks down S-adenosylhomocysteine to give adenosine and homocysteine.

SCID: Severe combined immunodeficiency, a group of rare congenital disorders characterized by severe impairment of immunity and absence of T lymphocytes.

Spleen: A large gland-like organ in the upper part of the abdominal cavity on the left side near the stomach. The largest structure in the lymphoid system, it has a number of immune functions, including the production of lymphocytes and plasma cells.

Stem cell: An unspecialized cell that is used to grow differentiated or specialized cells.

Thrombocythemia: An increase in the number of platelets circulating in the blood.

Thrombocytopenia: A persistent decrease in the number of platelets circulating in the blood.

T lymphocytes: The cells primarily responsible for cell-mediated immunity.

Thrush: A disease that is caused by a fungus of the genus Candida (C. albicans), occurs especially in infants and children, and is marked by white patches in the mouth.

Thymic shadow: The visualization of the thymus gland on an x-ray.

Thymus: A gland composed of largely lymphoid tissue that functions in cell-mediated immunity as the site where T cells develop. Present in children typically in the upper anterior chest or at the base of the neck. Tends to disappear or become rudimentary (vestigial) in the adult.

Toxic/toxicity: The quality of being poisonous.

Treatment: The management and care of a patient for the purpose of combating disease or disorder.

Urticaria: A vascular reaction in the upper dermis also known as hives.

Wild-type: Refers to a typical strain, phenotype, or gene occurring in a natural population, and therefore designated as representative of the group.

X chromosome: A sex chromosome that usually occurs paired in each female cell and single in each male cell.

X-linked recessive: Inherited through the X chromosome.

X-SCID: The most common form of SCID, the gene associated with this condition is located on the X chromosome.