In Vitro - experiments which are carried out in a laboratory as distinct from in vivo experiments

    In Vivo -experiments which are carried out in live animals as distinct from in vitro experiments

    Immunity¹ - The body response to a foreign antigen, either ingested as food, or as part of a foreign organism. There are two major ways the body defends itself; one is by antibody, production, the so called humeral response, as the antibodies circulate in the blood and the fluid between cells. The other is the cellular² response, as it involves the cells of the immune system, the family of leucocytes. The particular leucocyte³ responsible for immune specificity is the lymphocyte. In total cell mass there are as many lymphocytes as there are liver or brain cells. During development there are millions of B (from the Bone marrow⁴) lymphocytes made, each with a different cell membrane⁵ ligand, specific for any one of millions of antigens. The lymphocytes are circulating all the time so that they can have the chance to meet up with a foreign antigen. As soon as an antigen has been recognised by one of these cells, and bound to the cell ligand, it stimulates⁶ the cell to reproduce millions of copies of itself. All the daughter cells are clones of the original cell. These B lymphocyte daughters, migrate to the site where the antibody is needed. Instead of making an antigen for the membrane these cells make large amounts of soluble⁷ antibody. They are now recognisable as plasma⁸ cells. T lymphocytes ( having spent time in the Thymus) comprises the cell mediated⁹ response to an antigen. They are of two types, Killer¹⁰ T cells and Helper T Cells. Most T lymphocytes are helpers and they regulate the response of the B lymphocytes. The killer T cells are however capable of recognising the foreign antigen on the surface of a cell, and then killing¹¹ the entire cell. The immune response is part of a less specific defense¹² and healing response of the body known as inflammation.

    Indirect pulp¹³ cap - a dressing¹⁴, usually calcium¹⁵ hydroxide, placed against the pulpal wall of a deep cavity, in order to encourage affected¹⁶ dentine to remineralise. The cavity is closed with a temporary filling material and re-opened after 6 weeks to assess the state of the pulp.

    Induction¹⁷ - cell differentiation¹⁸ which is brought about by the influence of cytokines released by cells of another type.

    Infected dentine- dentine which has been damaged beyond repair by caries and is infected by large numbers of caries bacteria.

    Inflammation - is a whole complex of events which occur in sequence, in response to injury. Tissue damaged by bacteria, chemicals, heat, trauma¹⁹ etc, release histamine and bradykinin and serotonin which cause an increase in capillary²⁰ permeability and vasodilation. Both these factors contribute to the formation of a fluid exudate in the damaged tissue, which includes fibrinogen and therefore soon clots²¹ into a firm gel. This process has the effect of walling off the bacteria or toxic²² substances causing the damage, or at least it slows down their spread into surrounding tissues. Local macrophages, begin their phagocytic activity but their numbers are small. Damaged tissues also release interleukin, messengers which are transported all the way to the bone marrow, where millions of leucocytes are stored.. These stores now release leucocytes, mostly neutrophils into the blood. The neutrophils gather at the site of damage because the endothelial cells of the local capillary walls have become sticky to leucocytes. This stickiness is specific for leucocytes and is the work of selectins expressed on the cell membrane of the endothelial cell. The leucocytes begin to catch and roll along the endothelium until they are brought to a standstill. The increased permeability of the endothelial cells allows leucocytes to wriggle²³ out of the capillary and migrate into the damaged area. This migration²⁴ is also dependent on a process know as chemotaxis, in which cytokine messages from the damaged cells attract the leucocytes to come to their aid. After several days the battle zone is filled with dead bacteria, dead tissue cells, dead neutrophils and macrophages. This dead mass of tissue is called pus. The end of the event may be the gradual resorption of pus by fresh macrophages, or the pus, now under some pressure, may force its way somewhere else. Pus from the apex²⁵ of a tooth may escape laterally²⁶ through the alveolar bone and mucosa, where it is recognisable as a "gum boil".Ten days after a foreign protein is detected for the first time, the bodies immune system has mounted a more specific defense. Antibodies are produced by B lymphocytes and T lymphocytes have been alerted to the invasion.

    Insulin - an endochrine hormone²⁷ produced in the spleen which controls the amount of sugar in the blood by a) transporting it into cells and promoting glycolysis b) converting it into glycogen for storage in the liver and muscles, and c) converting it into fats. Without sufficient insulin, glucose²⁸ accumulates in the blood and urine and the cells of the body are starved, a condition known as diabetes²⁹. The control of insulin production is another example of a feedback system.

    Insulin-like growth factor (IGF) - a cytokine that influences growth hormone activity.

    Integrins are the cell surface receptor molecules³⁰ which match up with parts of the matrix protein ligands to allow adhesion of the cell to the matrix. The attachment³¹ of cells to matrix proteins also influences the cells behaviour, by the expression of genes³². The integrins are a family of proteins, found doing the same job on all animal cells. Their importance in maintaining the structural³³ integrity of cells led to the name integrins.

    Intercalated ducts - ducts which carry saliva³⁴ between the tubules and the striated³⁵ ducts.

    Interferon - a glycoprotein produced by cell s which mobilise the T lymphocytes to inhibit³⁶ viruses and the growth of cancer cells.

    Interleukins - a variety of compounds(about 20) that are produced by lymphocytes, macrophages, and monocytes. They regulate the cell mediated response of the immune system. Interleukin-1 is involved in the triggering of the immune response, starting acute inflammation and maintaining chronic³⁷ inflammation. Interleukin-2 is produced by helper T cells and induces proliferation of immune cells, both T and B. Interleukin-3 promotes the differentiation and proliferation of stem cells of the leucocyte family.Interleukin -6 produced by various cells including tumour³⁸ cells and acts as a stimulant³⁹ of plasma proteins and B and T cells. Interleukin -12 is produced by a range of cells. It activates⁴⁰ T cells and natural killer cells. It promotes the response to a range of pathogens including HIV of Interleukin-2. It appears to be one of the most promising⁴¹ Interleukins for the control of viral, bacterial⁴² and protozoal infections.

    Intermediate filaments⁴³ - unlike microfilaments and microtubules, they are verystable. Instead of being stacked proteins, as in actin, intermediate filaments are built of interlocking proteins. A dense⁴⁴ sheet of intermediate filaments strengthens the nucleus⁴⁵. Skin cells are filled with keratin, which at the last moment, just before they die. they cross link, to provide a really insoluble barrier layer of the skin. The cross linkage⁴⁶ is between the sulphur atoms of cysteine, one of keratin's amino acids.

    Interproximal wear - loss of enamel⁴⁷ on the adjacent surfaces of teeth which is due to continual friction⁴⁸ between the two surfaces as teeth move against each other.

    Intratubular dentine - dentine formed inside the tubule by the odontoblast process in response to tooth wear, ageing or arrested caries.

    Intrinsic fibres - refers to those fibres of cementum which were laid down by cementoblasts.

    Ionised - the loss or gain of an electron from an atom which makes it no longer neutral but an electrically charged ion. If the electron leaves the atom it becomes a positively⁴⁹ charged ion, such when calcium or sodium⁵⁰ becomes ionised (Ca+, Na+). If the electron is gained, the atom becomes relatively⁵¹ negatively charged such as when chlorine or a phosphate group of atoms lose an electron (Cl-, PO4-). Ionized atoms or groups of atoms are more reactive than when they are neutral.

    Ions - an atom or molecules which has a net electrical charge This may be caused by the temporary loss (positive ion) or gain (negative ion) of an electron. A calcium ion is written Ca+.

    Ipsilateral - the same side as distinct from contralateral. Often used to refer to the teeth, joint⁵² or muscles on the same side as chewing is occurring.