Immunology, antibodies and immunoassays: What are they?
Published Mar 16, 2021 • By Clémence Arnaud
Autoimmune diseases are a class of conditions that have very different consequences for the people who are affected by them. Approximately 5-8% of the world's population lives with an autoimmune disease, with a high prevalence in women (8 out of 10 people affected by an autoimmune disease are women).
How are autoimmune diseases defined? What is an antibody? What are immunoassays? For which health conditions do these tests already exist?
We explain it all in our article!
Immunology and autoimmune diseases: definition
The immune system is a network of biological processes that protects us from diseases. In the human body, it is capable of distinguishing what is non-self (foreign) from what is self and of then triggering a response to eliminate potentially dangerous cells. It can also eliminate abnormal cells from the body. This system is composed of two major subsystems: the innate immune system (which reacts with a preconfigured response without the body being aware of the antigen) and the adaptive immune system (also called the acquired immune system, which reacts with a tailored response to each foreign body by learning to recognize antigens it has previously encountered).
Autoimmune diseases occur when the immune system attacks a person's organs instead of protecting them. The elements of our immune system (antibodies, cytokines, etc.) will cause various lesions of varying severity depending on the disease. More than 80 health conditions are part of this classification, including cancer, rheumatoid arthritis, multiple sclerosis, lupus, etc.
Antibodies (Ab) are proteins whose role is to defend us against attacks on our body, and in particular, against viruses and bacteria. Immunoglobulin G (IgG) is the most common antibody in the human body.
These antibodies, also known as immunoglobulins, are produced by the immune system and in particular by cells called plasma B lymphocytes.
These plasma B cells, in response to the invasion of a foreign body (antigen) into the body, will produce antibodies which attack, neutralize and facilitate the elimination of the antigen in a specific way. Each antibody has a specificity for a particular antigen and keeps it in memory. This means that they can be produced more quickly in the event of a new attack by the same antigen.
In autoimmune diseases, autoantibodies target the individual's own cells. They are not 100% specific to a given disease and cannot be used alone to diagnose the condition.
Principle of immunoassays
These tests look for autoantibodies present in autoimmune diseases, but also in other health conditions.
Immunofluorescence assay (IFA)
This is a standard laboratory technique used to identify the presence of antibodies by their specific ability to react with antigens expressed in infected cells. The specific antibodies targeted are labelled with a compound that causes them to glow an apple-green color when observed under ultraviolet light through a microscope. This method applies to healthy cells but also to the detection of pathogenic cells.
Enzyme-linked immunosorbent assay (ELISA)
There are multiple types of ELISA test, but the three common types include:
- Direct ELISA: In this method, an antigen is immobilized on a test plate beforehand and a primary antibody with an attached (conjugated) enzyme is introduced, which binds to test antigen. Then, a substrate is added and changes color in reaction to the enzyme. This color is proportionate to the amount of antigen.
- Indirect ELISA: In this method, the antigen is present in the sample being tested. A primary conjugated antibody is added and binds to the antigen. An enzyme is then added and the subsequent addition of substrate creates a color reaction proportional to the amount of antigen.
- Sandwich ELISA: This is the most common type of ELISA test. In this version, two specific antibodies are used to sandwich the antigen. A surface is first prepared upon which a known quantity of capture antibody is fixed. Then, the antigen-containing sample is introduced and is captured by the antibody. Next, a second antibody is added and also binds to the antigen (hence the "sandwich", the antigen is now trapped between two antibodies). Substrate is then introduced and is converted by the enzyme into a color, fluorescent or electrochemical signal proportional to the amount of antigen.
Use of immunoassays in screening for certain autoimmune diseases
Testing for colorectal cancer
For people aged 50 to 75, a screening test is offered as part of the screening recommended by the CDC and the U.S. Preventative Services Task Force (USPSTF). This screening consists of carrying out a test at home, every 2 years, to look for blood in the stool.
If blood is detected in the stool, a colonoscopy is performed. This can diagnose colorectal cancer at an early stage, or even prevent cancer by detecting polyps or adenomas before they develop into cancerous lesions.
Testing in rheumatoid arthritis
In rheumatoid arthritis, autoantibodies appear several years before the first clinical symptoms. These autoantibodies are called anti–cyclic citrullinated peptide (anti-CCP) antibodies.
Biological tests measure inflammation markers (ESR and CRP) and look for the presence of these autoantibodies. Other factors, called rheumatoid factors, are also tested.
Rheumatoid factor is an immunoglobulin, often of the IgM type, which works against immunoglobulin G. It is found in 80% of cases of RA that have been evolving for several years, but only in 30% of early RA cases.
This factor is not specific (60-80% specificity) to rheumatoid arthritis, which means that it can be present in other illnesses. It may be present in a number of infectious diseases, autoimmune diseases (10-20% of lupus patients, >90% of patients with Sjögren's syndrome, etc.) or in healthy individuals, particularly the elderly.
Testing in lupus
In lupus, doctors use indirect immunofluorescence assay using HEp-2 cells to detect the antinuclear antibodies (ANAs) present in 99% of lupus cases.
Native anti-double strand DNA (dsDNA) antibodies are very specific for lupus, especially if they are at high levels. However, they are not constant, present in only 50-80% of cases.
Anti-ENA (extractable nuclear antigen) antibodies are most often directed against nucleocytoplasmic ribonucleoproteins and are detected through immunodiffusion using western blot assays. Anti-SM antibodies are described exclusively in lupus, but are present in only 10-20% of cases.
Anti-Ro/SSA and/or Anti-La/SSB autoantibodies are described in 30-70% of lupus cases, especially when skin lesions are present. They are not specific for lupus as they are also seen in 40-70% of primary Sjögren's syndrome cases.
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