Dictionary of terms


ABNORMAL A variation from the normal so-called genetic diseases resulting from abnormal genes.

ACUTE A descriptive term used to describe an illness, infection or symptom that is usually short in duration.

ADENOIDS Lymphoid tissue located in the upper part of the throat behind the nose.

ADENOSINE DEAMINASE (ADA) An enzyme found in lymphocytes (and other cells) responsible for removing certain of the toxins produced by their metabolism. In its absence lymphocytes fail to function, one of the causes of Severe Combined Immunodeficiency (SCID).

AGAMMAGLOBULINAEMIA Total lack of immunoglobulins (antibodies) in the blood.

ALLELES A person inherits two copies of any one gene: one from each parent. These copies may not be exactly the same! Each one is called an allele. So an allele is an alternative form of a gene.

AMINO ACID Any of a group of 20 molecules that are linked together to form proteins. Each protein is made up of a specific sequence of these molecules with its unique sequence coded for by DNA.

ANAEMIA Lower than normal numbers of red blood cells and/or low haemoglobin (oxygen-carrying pigment) concentration within red blood cells.

ANTIBODIES (IMMUNOGLOBULINS) Protein molecules that are produced by certain types of white cells (B-lymphocytes) in response to stimulation by an antigen. Their primary function is to fight bacteria, viruses, toxins and other substances foreign to the body.

ANTIGEN Any foreign substance that provokes an immune response when introduced into the body: the immune response usually involves both B-Lymphocytes and T-Lymphocytes.

ANTIGEN-PRESENTING CELL A group of cell types, which are able to break, down antigen and present it in a form able to stimulate lymphocytes.

AUTOANTIBODY An antibody that reacts against a person’s own tissue instead of against foreign substances.

AUTOIMMUNE DISEASE A disease that results when an individual develops antibodies which attack his own tissues.

AUTOLOGOUS Donated blood for your own use.

AUTOSOMAL Related to the autosome, that is any of the chromosomes other than the sex chromosomes (X and Y). Related to an abnormal gene it means males or females can be affected equally.

AUTOSOMAL DOMINANT In this situation, where an individual possesses an abnormal gene the features of the disorder related to that abnormal gene will be demonstrated. The condition will be transmitted to children with a 50% chance of an affected child for each pregnancy.

AUTSOMAL RECESSIVE In this situation the presence of an abnormal gene will not affect the individual him or herself. However, when two carriers of the same abnormal gene have children there is a 25% chance of an affected child for each pregnancy.

AUTOSOME Any chromosome other than the sex chromosome.


BACTERIA Single cell organisms (microorganisms) which can be seen only under a microscope. While some bacteria can be useful, many bacteria can cause disease in man.

BASE PAIRS In the DNA molecule, individual bases on each backbone match up with a specific base on the other strand, so that pairs form: G with C and A with T. These base pairs are found in the middle of the DNA double helix, like rungs on a twisted

B-CELL (B-LYMPHOCYTES) Cells of the immune system derived from bone marrow and involved in the production of antibodies.

BIOLOGY Study of living organisms.

BIOPSY Removal of a small piece of tissue for examination under a microscope.

BONE MARROW Soft tissue located in the hollow centres of most bones that contains developing blood cells and cells of the immune system

BONE MARROW TRANSPLANTATION Transfer of bone marrow, obtained by aspiration usually from the hip bones, from a donor – either related or unrelated – to a recipient. The donor bone marrow replaces the recipient bone marrow and provides a new immune system, curing the immunodeficiency.

BRONCHIECTASIS A dilatation of the tubes (bronchi) which lead to the air sacs of the lung; this is usually the consequence of recurrent infections.

BRUTON'S TYROSINE KINASE (BTK) The enzyme deficient in X-linked Agammaglobulinaemia. It is vital to the proper development of B-Lymphocytes.


CARRIER An individual who carries the abnormal gene for a specific condition without symptoms (such individuals are also referred to as heterzygotes and the genes involved as recessive).

CD (CLUSTERED DETERMINANT) Shorthand classification of the chemicals (molecules) found on the surface of leukocytes.

CD40 A molecule expressed on the surface of B-lymphocytes (and other cells) through which TLymphocytes influence their activity, particularly their ability to switch to producing immunoglobulin, other than IgM.

CD40 LIGAND A protein normally found on the surface of T-cells, which signals to B cells via CD40 and which is defective in X-linked Hyper IgM syndrome.

CELL The smallest part of any living thing. We have millions of cells which do many different jobs.

CHEMISTRY The science of the composition of substances and their effects upon one another.

CHORIONIC VILLUS SAMPLING A method of collecting the chorion cells from a pregnant mother. Chorion cells are situated on the wall of the uterus (womb). They are derived from the baby and can, therefore, be used to detect genetic abnormalities as well as other faults in the baby itself.

CHROMOSOME A structure present in the nucleus of all body cells (apart from red blood cells) which stores the genetic information (code). Normally humans have 23 pairs.

CHRONIC A descriptive term used to describe an illness or infection that may be recurrent or lasting a long time.

CIRRHOSIS Scarring of the liver as a result of previous inflammation.

CLONING The process by which a culture is grown and amplified to produce multiple copies of a single molecule.

CLUSTERED DETERMINANT (CD) Shorthand classification of the chemicals (molecules) found on the surface of leukocytes.

COMPLEMENT A complex series of blood proteins that act in a definite sequence to bring about the destruction of bacteria, viruses and fungi.

CRYPTOSPORIDIOSIS An unusual infection, which causes diarrhea and may become chronic, particularly residing in the gastro-intestinal tract and bile ducts. It can be caused through water contamination, uncooked or unwashed food, handling farm animals or household pets and through touch by contaminated hands. A minority of people with primary immunodeficiencies are at special risk from this infection.

CYTOKINES A group of chemicals secreted by cells, which organise the immune, and inflammatory response by which we defend ourselves against infection. Lymphokines represent the cytokines secreted by leukocytes as part of this process. Examples of cytokines are interleukins, interferons and colony stimulating factors.

CYTOTOXIC Harmful to cells.


DELETION The absence of genetic material on a chromosome.

DNA Deoxyribonucleic Acid. The chemical part of chromosomes which provides the genetic information (code).

DNA REPLICATION The process whereby a copy of the DNA within the nucleus is made. Once the DNA is replicated (copied exactly), cell division can proceed. DNA replication allows each of the new cells to have the same genetic information.

DOMINANT GENE If a faulty gene is dominant, it will show an effect even though there is a working copy of the gene on the other chromosome. A person only needs to inherit a faulty gene for a dominantly inherited condition, from one parent, to develop a disease.

DOMINANT ALLELE A dominant allele is one whose effect on the cell is unhindered by the presence of another allele.

DOUBLE HELIX Two intertwining spirals. The best known example of a double helix is the DNA molecule.


ENZYME Protein that acts as catalyst (= makes work) to a specific biochemical reaction. Enzymes make the chemistry of cells work.


FAMILIAL An inherited characteristic of some or all members of a family.

FRAGILE SITE A site on a chromosome where DNA is prone to break. For example, in fragile X syndrome there is a fragile site on the female X chromosome
FUNGUS Member of a class of relatively primitive microorganisms, including mushrooms, yeasts and moulds.


GAMMABLOBULIN The blood serum protein globulin (y-globulin) which includes the antibodies/immunoglobulin responsible for immune responses.

GENE Section of DNA on a chromosome that codes for a functional RNA molecule and thus a protein. Put another way, a word, rather than a letter in the genetic code.

GENE THERAPY Attempting to cure genetic diseases by placing a normal ‘healthy’ gene into cells that have a faulty version of that gene.

GENETIC CODE The way in which messages for protein are ‘written’ in DNA. DNA is made up of 4 distinct chemical bases strung along a sugar backbone. These bases act as the letters of a very short alphabet, the socalled genetic code.

GENETICIST An expert in the study of genes and heredity.

GENOME The complete complement of genes unique to a given species.

GLAND A collection of cells or an organ with a specialised ability to make and secrete chemical substances such as enzymes and hormones essential for the normal functioning of the body.

GLOBULIN Any group of globular proteins that are generally insoluble in water and present in blood, eggs, and milk and as a reserve protein in seeds.

GRANULOCYTE A white cell of the immune system characterised by the ability to phagocytize (take in or ingest) foreign material. A granulocyte is identified under the microscope by the presence of many granules. Neutrophils, eosinophils and basophils are examples of granulocytes.


HAEMOPOIETIC An agent that promotes the formation of blood cells.

HELPER T-CELLS A sub-set of T-cells that help B-cells and T-cells to function better.

HOMOZYGOTE Has a double dose of an abnormal gene as in affected individuals with an autosomal recessive disorder.

HORMONES Substances in humans, plants and animals that help cells to function properly.

HYPOGAMMAGLOBULINAEMIA Lower than normal levels of immunoglobulins (or antibodies) in the blood, causing the individual to be more prone to infections.

HISTOCOMPATIBILITY TESTS Immunologic tests for comparing the antigens on the tissue of a bone marrow transplant donor with those of the recipient (the person who is to receive it). The more closely matched the better the chance for the transplant to take.


IgA An immunoglobulin found in blood, tears, saliva, and in the mucous membranes of the respiratory, urogenital and intestinal tracts.

IgE An immunoglobulin found in tiny amounts in the blood and responsible for allergic reactions.

IgG The most abundant and common of the immunoglobulins found in the blood and tissue fluids. IgG functions mainly against bacteria and some viruses. It is the only antibody that can cross the placenta.

IgM An immunoglobulin found in the blood. IgM functions in much the same way as IgG but is formed earlier in the immune response. It is also very efficient in activating complement.

IMMUNE ENTEROPATHY Disturbance of the immune system within the gastro-intestinal tract which causes chronic diarrhea.
IMMUNE RESPONSE The activity or response of the immune system against foreign substances.

IMMUNOCOMPETENT Capable of developing an immune response.

IMMUNODEFICIENCY A state of either a congenital (present at birth) or an acquired abnormality of the immune system which prevents adequate immune responsiveness.

IMMUNOGLOBULIN (abbreviation: Ig) One of a group of proteins (globulins) in the body that act as antibodies. They are produced by specialist white blood cells (B-lymphocytes) and are present in blood serum and other body fluids. There are several different classes (IgA, IgE, IgG, IgM) all having different functions.

IMMUNOLOGY Study of immunity and all of the phenomena connected with the defense mechanism of the body.

IMMUNOSUPPRESSION The process of deliberately reducing the immune response, usually accomplished with medicine and used in the treatment of autoimmunity and to prevent rejection of a transplanted organ.

IN VITRO Outside of a living environment; refers to a process of study taking place in test tubes, etc.

IN VIVO In the intact animal, usually refers to a process or study which takes place in a living animal

INHERITANCE The transmission of genetic information from parent to offspring.

INTERFERON One of a family of cytokines that have antiviral and immune modulating actions

INTERLEUKINS A group of chemicals involved in signaling between lymphocytes, antigen presenting cells and other cells in the body.

INTERLEUKIN 2 (IL2) Cytokine responsible for stimulating the growth and differentiation of T-Lymphocytes.

INTRAVENOUS (abbreviation IV) Inside or into a vein, e.g. immunoglobulin infusion  directly into a vein.


LEUCOCYTE (White Blood Cell, (WBC)) A group of small colourless blood cells that play a major role in the body’s immune system. There are five basic white blood cells: monocytes, lymphocytes, neutrophils, eosinophils and basophils.

LEUCODEPLETION Removing white cells from donated blood.

LYMPH A transparent, slightly yellow fluid, containing mainly lymphocytes. Lymph is composed of tissue fluids collected from all parts of the body and returned to the blood via the lymphatic vessels.

LYMPH NODES Small bean-sized organs of the immune system, distributed widely throughout the body. Each lymph node contains a variety of specialised compartments that house B-cells, T-cells, and macrophages. Lymph nodes unite in one location the several factors needed to produce an immune response.

LYMPHOCYTES Small white cells, normally present in the blood and in lymphoid tissue, that bear the major responsibility for carrying out the functions of the immune system. There are two major forms of lymphocytes, B-cells and T-cells, which have distinct but related functions in generating an immune response.

LYMPHOID TISSUES Tissues involved in the formation of lymph, lymphocytes, and antibodies. They consist of the lymph nodes, thymus, tonsils and spleen.


MACROPHAGE A phagocytic cell found in the tissues, able to destroy invading bacteria or other foreign material - see MONOCYTE.

MAJOR HISTOCOMPATIBILITY COMPLEX An important region on the 6th chromosome in man which controls the transplant rejection responses, and also the intensity of the immune response in general.

METABOLISM A general term which summaries the chemical changes within a cell, which result in either the building up or breaking down of living material.

METABOLITE A substance which is either acted upon or produced in the process of metabolism.

MICRO-ORGANISM (MICROBE) Any organism too small to be visible to the naked eye. They include bacteria, some fungi, protozoa and viruses.

MOLECULAR BIOLOGY Study of the molecules that are associated with living organisms, especially proteins and nucleic acids.

MOLECULE Smallest portion (of a substance) capable of existing independently with all its properties; small particle.

MONOCLONAL ANTIBODY Large protein molecule produced by white blood cells that seek out and destroy harmful foreign substances.

MONOCYTE Phagocytic cell found in the blood that acts as a scavenger, capable of destroying invading bacteria or other foreign material; these cells develop into macrophages in tissues.

MOSAICISM The situation in which a genetic or chromosomal abnormality does not occur in all body cells. The proportion of normal to abnormal can determine the severity of the disorder.

MUTANT A gene which has undergone change from the normal gene. A mutation may be established, passed through the generations of a family or be part of a new ‘sporadic’ mutation (i.e. occurring from time to time).

MUTATION A change in the structure of DNA. Such changes can be passed on to the next generation. A few mutations are beneficial, but others can cause genetic disorders.


NUCLEUS The central part of human, animal and plant cells; it controls a cell’s functions and contains the chromosomes.

NEUTROPHIL A type of granulocyte found in the blood and tissues that can take in and destroy microorganisms.


OPPORTUNISTIC INFECTION An infection in immunodeficient, or immunosuppressed persons, caused by organisms that do not usually affect people with normal immune systems.

OPSONIZATION (OPSONIN) A process by which phagocytosis (the work done by phagocytes) is made easier by depositing opsonins (e.g. complement and antibody) on the target of the immune system.

ORGANISM Any living thing, which may consist of a single cell (microorganism) or a group of differentiated but interdependent cells.


PARASITE A plant or animal that lives, grows, and feeds on or within another living organism.

PENETRANCE An individual carrying a dominant gene may show a variable degree of the symptoms of the disorder associated with that gene. The gene is said to display different degrees of penetrance.

PHAGOCYTES A general class of white blood cells that ingest microbes or other cells and foreign particles. Monocytes, macrophages and neutrophils are types of phagocytes.

PLASMA Liquid part of blood (i.e. excluding blood cells) which consists of water containing a large number of dissolved substances including proteins, salts (especially sodium and potassium chlorides and bicarbonates), food material (glucose, amino acids, fats) hormones, vitamins, and excretory materials.

PLASMA CELLS Antibody-producing cells descended from B-cells.

PLATELETS Tiny cell fragments which circulate in the bloodstream, and are important for preventing bleeding.

PNEUMOCOCCUS A type of streptococcal bacterium which is associated with pneumonia and other infections of the respiratory tract.

PNEUMOCYSTIS CARINII PNEUMONIA (PCP) An ‘opportunistic’ infection -- which does not usually cause illness except in people with defective immune systems - in this case defective T-cell function. PCP is a severe form of pneumonia.

PREDISPOSITION (Genetic) When for genetic reasons an individual is more prone to develop a particular disorder.

PROPHYLACTIC Medical therapy given to prevent or guard against disease and infection.

PROTEIN A class of chemicals found in the body made up of chains of amino acids (building blocks). Immunoglobulins (antibodies) are protein.

PROTOZOA A group of single-celled animals. Some are disease causing parasites of man.


RECEPTOR A structural protein on the cell surface that binds to specific factors such as hormones, antigens, or neurotransmitters.

RECOMBINANT Synthetic (recombined with something else).

RECOMBINANT DNA DNA produced by joining pieces of DNA from different sources.
RNA The chemical which transfers the genetic code from the nucleus of cells to the cytoplasm where proteins are synthesised.


SCLEROSING CHOLANGITIS A form of liver disease which is particularly associated with X-linked Hyper IgM syndrome.

SERUM Blood plasma from which the fibrin and clotting factors have been removed by centrifugation or vigorous stirring, so that it cannot clot. Serum containing specific antibodies or antitoxins may be used in the treatment or prevention of certain infections.

SPLEEN An organ in the abdominal cavity. It is an important site of antibody production and like lymph nodes contains B-cells, T-cells and macrophages.

SUBCUTANEOUS Anything relating to the loose cellular tissue beneath the skin, e.g. an immunoglobulin infusion directly into the tissue directly beneath the skin.

STEM CELLS Cells from which all blood cells and immune cells are derived.

STEROIDS A family of chemicals, some made by the adrenal glands, others synthesised. They include male and female sex hormones and also drugs that suppress inflammation and harmful immune responses.

SUPPRESSOR T-CELLS A sub-set of T-cells that “turn off” antibody production by B-cells.


T-CELLS (OR T-LYMPHOCYTES) Lymphocytes that are processed in the thymus, an organ in the chest. They are responsible, in part, for carrying out the immune response.

THROMBOCYTOPENIA Reduced numbers of platelets in the bloodstream.

THYMUS GLAND A lymphoid organ located behind the upper portion of the sternum (breastbone). The thymus is the chief "educator" of T-cells. This organ increases in size from infancy to adolescence, and then begins to shrink.

TOLERANCE Inability of the immune system to respond to a particular antigen, although it is able to respond adequately to other antigens.

TONSILS Two oval-shaped masses of lymphoid tissue situated on each side of the back of the mouth, leading to the throat.

TRANSGENIC A transgenic animal is one that has been genetically altered to have new and inheritable characteristics.


VASCULITIS Inflammation of the blood vessels. This may cause narrowing or blockage, thus restricting blood flow.

VIRUS Microbe (too small to be seen under a microscope) causing infectious disease. Can reproduce only in living cells.


X-LINKED Refers to the inheritance of a particular characteristic or disorder from a gene carried on the X (or female sex) chromosome. Also known as sexlinked inheritance.

X-LINKED RECESSIVE The form of inheritance where the abnormal gene is carried on the X chromosome. In this situation girls are usually the carriers and boys are affected. This is because the male Y-chromosome does not carry the compensatory normal gene, and the X chromosome is not paired in a male.