lgG level subsequently declines and rises again after endogenous synthesis begins in the infant. Due to low molecular weight, lgG can diffuse into the interstitial fluid and protect against tissue infection. lgG is highly effective against bacterial infection. In chronic hepatitis, there is a marked increase in plasma lgG.

Ig isotype
Class of antibody molecules that share a heavy chain Fc region. There are five lg isotypes: α (lgA), γ(lgG), δ(lgD), ε(lgE) and µ(lgM).

Ig superfamily
See immunoglobulin superfamily.

IgA
An immunoglobulin predominantly synthesised in the plasma cells underlying the mucosa of the gastrointestinal and respiratory tracts, the skin and exocrine glands. It is also found in the colostrum, sweat and tears. lgA is involved in protecting neonates from gastrointestinal infection.

Identity
The extent to which two nucleotide or amino acid sequences are invariant.

Ideogram
A diagrammatic representation of the G-banding pattern of a chromosome. See karyogram

Idiopathic
Of unknown cause.

Idiopathic haemochromatosis
A severe form of iron overload characterised by excessive intestinal iron absorption and deposition of iron in the skin and tissues.

IgM
A high molecular weight pentamer found mostly in the vascular space. It is the first immunoglobulin secreted by B lymphocytes stimulated to form plasma cells and is the first line of defence against invading organisms. IgM increases only after birth and high levels before birth indicate intrauterine infections such as congenital toxoplasmosis, rubella, cytomegalovirus, herpes simplex virus and syphilis. In primary biliary cirrhosis, plasma lgM is significantly elevated.

IGS
Internal guide sequence. See guide sequence.

Illegitimate base
A base other than A, C, G or T found in DNA. It may also be a normal base that may have been damaged or inappropriate bases such as uracil.

Illegitimate recombination
A type of recombination which requires little or no homology between the recombining partners and results from aberrant cellular processes including illegitimate end-joining and strand slippage or looping during replication. Unequal crossing over is described as illegitimate although the mechanism involves normal homologous recombination.

Image processing
Computer processing of images obtained from microscopy that reveals information not visible to the naked eye.

Imaging disc
 A group of cells that are set apart in the Drosophila embryo which subsequently develop into an adult structure such as leg or wing.

Imaging agents
Antibodies linked a chemical group to enable scanners to detect them easily. The proteins bind specifically to certain tissue types such as tumour cells thereby allowing differentiation of tumour from normal tissues.

Imidazole
The side chain group of histidine.

Imino acid
Contains secondary instead of primary amine function (e.g. proline).

Immediate early gene
A viral gene that is expressed immediately after infection.

Immediate-type hypersensitivity
Hypersensitivity reaction that occurs within minutes after the interaction of antigen and antibody.

Immobilised cells
Cells entrapped in matrices such as alginate, polyacrylamide and agarose that are designed for use in membrance and filter bioreactors.

Immobilised enzyme
An enzyme that is physically defined or localized in a defined region so that it can be reused.

Immortalization
The genetic transformation of a cell type into a cell line that can proliferate indefinitely.

Immortalizing oncogene
A gene which when transfected enables a primary cell in culture to grow indefinitely.

Immune adherence
The adherence of particulate antigen coated with C3b to cells with C3b receptors.

Immune complex
The complex of antigen bound to antibody.

Immune paresis  
Proliferation of a single clone of cells with suppression of the synthesis of immunoglobulins from other clones. As a result, the concentrations of immunoglobulins other than the paraprotein  are low which is apparent in the serum electrophoretic pattern as a reduced or absent band. Immune paresis may increase susceptibility to infection.

Immune response
The mechanisms used by vertebrates to respond to the presence of a foreign antigen.