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In
biochemistry
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
, immunostaining is any use of an
antibody
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
-based method to detect a specific
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
in a sample. The term "immunostaining" was originally used to refer to the
immunohistochemical staining
Immunohistochemistry (IHC) is the most common application of immunostaining. It involves the process of selectively identifying antigens (proteins) in cells of a tissue section by exploiting the principle of antibodies binding specifically to ant ...
of tissue sections, as first described by
Albert Coons
Albert Hewett Coons (June 28, 1912 – September 30, 1978) was an American physician, pathologist, and immunologist. He was the first person to conceptualize and develop immunofluorescent techniques for labeling antibodies in the early 1940 ...
in 1941.
However, immunostaining now encompasses a broad range of techniques used in
histology
Histology,
also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...
,
cell biology, and
molecular biology
Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. The study of chemical and physi ...
that use antibody-based staining methods.
Techniques
Immunohistochemistry
Immunohistochemistry or IHC staining of
tissue sections (or
immunocytochemistry
Immunocytochemistry (ICC) is a common laboratory technique that is used to anatomically visualize the localization of a specific protein or antigen in cells by use of a specific primary antibody that binds to it. The primary antibody allows visual ...
, which is the staining of
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
s), is perhaps the most commonly applied immunostaining technique.
While the first cases of IHC staining used
fluorescent
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, ...
dyes (see ''
immunofluorescence''), other non-fluorescent methods using
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
s such as
peroxidase
Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides.
Functionality
Peroxidases typically ca ...
(see ''
immunoperoxidase staining'') and
alkaline phosphatase are now used. These enzymes are capable of catalysing reactions that give a coloured product that is easily detectable by light
microscopy. Alternatively,
radioactive
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...
elements can be used as labels, and the immunoreaction can be visualized by
autoradiograph
An autoradiograph is an image on an X-ray film or nuclear emulsion produced by the pattern of decay emissions (e.g., beta particles or gamma rays) from a distribution of a radioactive substance. Alternatively, the autoradiograph is also available ...
y.
[Immunohistochemistry Introduction](_blank)
/ref>
Tissue preparation or ''fixation'' is essential for the preservation of cell morphology and tissue architecture. Inappropriate or prolonged fixation may significantly diminish the antibody binding capability. Many antigens can be successfully demonstrated in formalin
Formaldehyde ( , ) ( systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section F ...
-fixed paraffin Paraffin may refer to:
Substances
* Paraffin wax, a white or colorless soft solid that is used as a lubricant and for other applications
* Liquid paraffin (drug), a very highly refined mineral oil used in cosmetics and for medical purposes
* Alkan ...
-embedded tissue sections. However, some antigens will not survive even moderate amounts of aldehyde fixation. Under these conditions, tissues should be rapidly fresh frozen in liquid nitrogen and cut with a cryostat. The disadvantages of frozen sections include poor morphology, poor resolution at higher magnifications, difficulty in cutting over paraffin sections, and the need for frozen storage. Alternatively, vibratome
A vibratome is an instrument used to cut thin slices of material (although, usually thicker slices than those cut in paraffin-embedded samples using a microtome). It is similar to a microtome but uses a vibrating blade to cut through tissue. Th ...
sections do not require the tissue to be processed through organic solvents or high heat, which can destroy the antigenicity, or disrupted by freeze thawing. The disadvantage of vibratome sections is that the sectioning process is slow and difficult with soft and poorly fixed tissues, and that chatter marks or vibratome lines are often apparent in the sections.
The detection of many antigens can be dramatically improved by ''antigen retrieval'' methods that act by breaking some of the protein cross-links formed by fixation to uncover hidden antigenic sites. This can be accomplished by heating for varying lengths of times (heat induced epitope retrieval or HIER) or using enzyme digestion (proteolytic induced epitope retrieval or PIER).
One of the main difficulties with IHC staining is overcoming specific or non-specific background. Optimisation of fixation methods and times, pre-treatment with blocking agents, incubating antibodies with high salt, and optimising post-antibody wash buffers and wash times are all important for obtaining high quality immunostaining. In addition, the presence of both positive and negative controls for staining are essential for determining specificity.
Flow cytometry
A flow cytometer
Flow cytometry (FC) is a technique used to detect and measure physical and chemical characteristics of a population of cells or particles.
In this process, a sample containing cells or particles is suspended in a fluid and injected into the fl ...
can be used for the direct analysis of cells expressing one or more specific proteins. Cells are immunostained in solution using methods similar to those used for immunofluorescence, and then analysed by flow cytometry.
Flow cytometry has several advantages over IHC including: the ability to define distinct cell populations by their size and granularity; the capacity to gate out dead cells; improved sensitivity; and multi-colour analysis to measure several antigens simultaneously. However, flow cytometry can be less effective at detecting extremely rare cell populations, and there is a loss of architectural relationships in the absence of a tissue section. Flow cytometry also has a high capital cost associated with the purchase of a flow cytometer.
Western blotting
Western blotting allows the detection of specific proteins from extracts made from cells or tissues, before or after any purification steps. Proteins are generally separated by size using gel electrophoresis before being transferred to a synthetic membrane
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. B ...
via dry, semi-dry, or wet blotting methods. The membrane can then be probed using antibodies using methods similar to immunohistochemistry, but without a need for fixation. Detection is typically performed using peroxidase
Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides.
Functionality
Peroxidases typically ca ...
linked antibodies to catalyse a chemiluminescent
Chemiluminescence (also chemoluminescence) is the emission of light (luminescence) as the result of a chemical reaction. There may also be limited emission of heat. Given reactants A and B, with an excited intermediate ◊,
: + -> lozenge - ...
reaction.
Western blotting is a routine molecular biology method that can be used to semi-quantitatively compare protein levels between extracts. The size separation prior to blotting allows the protein molecular weight
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
to be gauged as compared with known molecular weight markers.
Enzyme-linked immunosorbent assay
The enzyme-linked immunosorbent assay or ELISA is a diagnostic method for quantitatively or semi-quantitatively determining protein concentrations from blood plasma
Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but contains proteins and other constituents of whole blood in suspension. It makes up about 55% of the body's total blood volume. It is the intr ...
, serum or cell/tissue extracts in a multi-well plate format (usually 96-wells per plate). Broadly, proteins in solution are absorbed to ELISA plates. Antibodies specific for the protein of interest are used to probe the plate. Background is minimised by optimising blocking and washing methods (as for IHC), and specificity is ensured via the presence of positive and negative controls. Detection methods are usually colorimetric or chemiluminescence based.
Immuno-electron microscopy
Electron microscopy or EM can be used to study the detailed microarchitecture of tissues or cells. Immuno-EM allows the detection of specific proteins in ultrathin tissue sections. Antibodies labelled with heavy metal particles (e.g. gold) can be directly visualised using transmission electron microscopy
Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...
. While powerful in detecting the sub-cellular localisation of a protein, immuno-EM can be technically challenging, expensive, and require rigorous optimisation of tissue fixation and processing methods. Protein biotinylation
In biochemistry, biotinylation is the process of covalently attaching biotin to a protein, nucleic acid or other molecule. Biotinylation is rapid, specific and is unlikely to disturb the natural function of the molecule due to the small size of bi ...
''in vivo'' was proposed to alleviate the problems caused by frequent incompatibility of antibody staining with fixation protocols that better preserve cell morphology.
Methodological overview
In immunostaining methods, an antibody
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
is used to detect a specific protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
epitope
An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The epitope is the specific piece of the antigen to which an antibody binds. The p ...
. These antibodies can be ''monoclonal
Monoclonality refers to the state of a line of cells that have been derived from a single clonal origin. Thus "monoclonal cells" can be said to form a single clone. The term ''monoclonal'' comes from the Ancient Greek ''monos'', meaning "alone" o ...
'' or ''polyclonal
Polyclonal B cell response is a natural mode of immune response exhibited by the adaptive immune system of mammals. It ensures that a single antigen is recognized and attacked through its overlapping parts, called epitopes, by multiple clones of ...
''. Detection of this first or ''primary antibody'' can be accomplished in multiple ways.
* The primary antibody can be directly labeled using an enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
or fluorophore
A fluorophore (or fluorochrome, similarly to a chromophore) is a fluorescent chemical compound that can re-emit light upon light excitation. Fluorophores typically contain several combined aromatic groups, or planar or cyclic molecules with se ...
.
* The primary antibody can be labeled using a small molecule which interacts with a high affinity binding partner that can be linked to an enzyme or fluorophore. The biotin- streptavidin is one commonly used high affinity interaction.
* The primary antibody can be probed for using a broader species-specific ''secondary antibody
Primary and secondary antibodies are two groups of antibodies that are classified based on whether they bind to ''antigens or proteins'' directly or target another (primary) antibody that, in turn, is bound to an ''antigen or protein''.
Primary
...
'' that is labeled using an enzyme, or fluorophore.
* In the case of electron microscopy, antibodies are linked to a heavy metal particle (typically gold nanoparticles in the range 5-15nm diameter).
As previously described, enzymes such as horseradish
Horseradish (''Armoracia rusticana'', syn. ''Cochlearia armoracia'') is a perennial plant of the family Brassicaceae (which also includes mustard, wasabi, broccoli, cabbage, and radish). It is a root vegetable, cultivated and used worldwide ...
peroxidase
Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides.
Functionality
Peroxidases typically ca ...
or alkaline phosphatase are commonly used to catalyse reactions that give a coloured or chemiluminescent
Chemiluminescence (also chemoluminescence) is the emission of light (luminescence) as the result of a chemical reaction. There may also be limited emission of heat. Given reactants A and B, with an excited intermediate ◊,
: + -> lozenge - ...
product. Fluorescent
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, ...
molecules can be visualised using ''fluorescence microscopy'' or ''confocal microscopy
Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM) or laser confocal scanning microscopy (LCSM), is an optical imaging technique for increasing optical resolution and contrast of a micrograph by means of using a s ...
''.
Applications
The applications of immunostaining are numerous, but are most typically used in ''clinical diagnostics'' and ''laboratory research''.
Clinically, IHC is used in histopathology for the diagnosis of specific types of cancers based on molecular markers.
In laboratory science, immunostaining can be used for a variety of applications based on investigating the presence or absence of a protein, its tissue distribution, its sub-cellular localisation, and of changes in protein expression or degradation.
See also
* Cutaneous conditions with immunofluorescence findings
Several cutaneous conditions can be diagnosed with the aid of immunofluorescence studies.
Cutaneous conditions with positive direct or indirect immunofluorescence when using salt-split skin include:
For several subtypes of pemphigus a variety ...
* Immunostaining protocol
*
References
{{Protein methods
Immunology
Flow cytometry
Protein methods