In molecular biology

Molecular biology

Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

, an intrabody (from
intracellular

Intracellular

Not to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...

and antibody

Antibody

An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...

) is an antibody that works within the cell to bind to an intracellular protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

. Due to the lack of a reliable mechanism for bringing antibodies into the cell from the extracellular environment, this typically requires the expression of the antibody within the target cell, which can be accomplished by gene therapy

Gene therapy

Gene therapy is the insertion, alteration, or removal of genes within an individual's cells and biological tissues to treat disease. It is a technique for correcting defective genes that are responsible for disease development...

. As a result, intrabodies are defined as antibodies that have been modified for intracellular localization, and the term has rapidly come to be used even when antibodies are produced in prokaryote

Prokaryote

The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...

s or other non-target cells. This term can apply to several types of protein targeting

Protein targeting

Protein targeting or protein sorting is the mechanism by which a cell transports proteins to the appropriate positions in the cell or outside of it. Sorting targets can be the inner space of an organelle, any of several interior membranes, the cell's outer membrane, or its exterior via secretion...

: the antibody may remain in the cytoplasm

Cytoplasm

The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

, or it may have a nuclear localization signal

Nuclear localization signal

A nuclear localization signal or sequence is an amino acid sequence which 'tags' a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysines or arginines exposed on the protein surface. Different...

, or it may undergo cotranslational translocation across the membrane into the lumen of the endoplasmic reticulum

Endoplasmic reticulum

The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...

, provided that it is retained in that compartment through a KDEL

KDEL (amino acid)

KDEL is a sequence in the amino acid structure of a protein which keeps it from secreting from the endoplasmic reticulum . It also targets proteins from other locations to the ER...

 sequence.

Because antibodies ordinarily are designed to be secreted from the cell, intrabodies require special alterations, including the use of single-chain antibodies (scFvs), modification of immunoglobulin VL domains for hyperstability, selection of antibodies resistant to the more reducing intracellular environment, or expression as a fusion protein with maltose binding protein

Maltose binding protein

Maltose-Binding Protein is a part of the maltose/maltodextrin system of Escherichia coli, which is responsible for the uptake and efficient catabolism of maltodextrins. It is a complex regulatory and transport system involving many proteins and protein complexes...

 or other stable intracellular proteins. Such optimizations have improved the stability and structure of intrabodies, allowing the publication of a variety of promising applications against hepatitis B, avian influenza, prion

Prion

A prion is an infectious agent composed of protein in a misfolded form. This is in contrast to all other known infectious agents which must contain nucleic acids . The word prion, coined in 1982 by Stanley B. Prusiner, is a portmanteau derived from the words protein and infection...

 diseases, inflammation

Inflammation

Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

, Parkinson's disease

Parkinson's disease

Parkinson's disease is a degenerative disorder of the central nervous system...

, and Huntington's disease

Huntington's disease

Huntington's disease, chorea, or disorder , is a neurodegenerative genetic disorder that affects muscle coordination and leads to cognitive decline and dementia. It typically becomes noticeable in middle age. HD is the most common genetic cause of abnormal involuntary writhing movements called chorea...

.