This glossary contains the definitions of some of the technical terms or words associated with COVID-19, SARS-CoV-2, or immunology. If you have a suggestion of a new word or term to include, please get in touch by emailing us at UK-CIC@immunology.org.
A
ACE2 receptor – Angiotenin-converting enzyme 2 (ACE2) is an enzyme found on the outer surface of cells in the lungs, arteries, heart, kidney and intestines. It can be used as an entry point to our cells by some coronaviruses, including SARS-CoV-2, which can attach to this receptor.
Adaptive immune response – One of the two main ways the immune system fights off infection from pathogens, along with the innate immune response. It is a part of the immune system made up of specialised cells and processes, that removes pathogens and prevents their spread in the body. Unlike the innate immune response, which uses a broad approach, the adaptive immune response is highly specific to each pathogen encountered, and so generally takes longer to take effect after initial exposure.
Alpha variant - The B.1.1.7 variant of SARS-CoV-2, formerly known as the 'Kent' variant as this is where it was first identified. The World Health Organisation recently announced a new naming system for variants of concern and variants of interest using the Greek alphabet, with names that are easy to pronounce and non-stigmatising.
Antibodies – Y-shaped proteins produced by B cells. They stick to invading pathogens, such as SARS-CoV-2, and mark them for destruction. They can also signal to other cells to help them recognise pathogens.
Antigen – a unique molecule on a pathogen that triggers an immune response. Part of this response can be to produce antibodies against it. Antibodies are produced specifically to match each antigen.
Antiviral response – The response by the immune system to rid the body of a virus. This involves several mechanisms and types of cells, including the innate and adaptive immune system, T cells, B cells, and antibodies.
Assay – A specific laboratory test to assess the immunological activity of a biological sample, for example to see whether a specific T cell type is present in blood and how much of it.
Asymptomatic – If someone is carrying a virus, and tests positive for it, but does not show any symptoms of the relevant disease (e.g. cough, fever), they are ‘asymptomatic’.
B
B cell – a type of white blood cell that produces antibodies as part of the adaptive immune system.
Beta variant - The B.1.351 variant of SARS-CoV-2, formerly known as the 'South African' variant as this was where it was first identified. The World Health Organisation recently announced a new naming system for variants of concern and variants of interest using the Greek alphabet, with names that are easy to pronounce and non-stigmatising.
Biomarkers/biological markers – A measurable factor that is linked to a specific aspect of health or the immune system. It can be anything that can be used to indicate a particular disease state. For example, measuring levels of a specific protein may indicate how severely a patient may be affected by a disease.
Blood plasma – The clear, yellow liquid part of the blood that is left after removing red blood cells, white blood cells, and platelets. It contains important proteins including enzymes, antibodies, and clotting factors, and makes up roughly 55% of the total volume of blood.
Blood serum – Blood plasma without clotting factors. Blood serum contains proteins, electrolytes, antibodies, antigens, hormones, and many other cell types, but does not contain red or white blood cells. Immunologists can use blood serum samples to test for levels of these molecules.
C
Cell membrane – The biological barrier between the inside and outside of a cell. It controls the movement of substances in and out of a cell, and often contains receptors that perform a variety of functions as part of the immune system.
Co-morbidity – The presence of one or more additional medical conditions in the same person at the same time. A number of other medical conditions (e.g. type 2 diabetes, heart disease) are being investigated as comorbidities in patients with COVID-19, where they might increase the chance of severe symptoms or death.
Cohort – A group of people, patients or participants in a study that have a shared characteristic. For example, cohorts of healthcare workers have been studied as part of several COVID-19 studies due to their relatively high levels of exposure to SARS-CoV-2.
Complement/the complement system – Also called the complement cascade, this is a part of the immune system including over 30 proteins that work in harmony to eliminate pathogens. When the cascade is activated it “complements” the role of antibodies and phagocytes. It can drive inflammation and kill cells.
Convalescence/convalescent – The gradual recovery of health and strength after suffering from a disease. Someone who is recovering from COVID-19 is “in convalescence” or is convalescent, in the same way you might describe a patient as “in recovery”.
Correlates of protection – Specific immune markers, or measurable signs, that a person is immune to becoming infected with a particular bacteria or virus.
Corticosteroids – A type of steroid used in medicine to treat a wide range of conditions. An example is dexamethasone, which is used to treat COVID-19. Corticosteroids also exist naturally in the body as a part of the immune response and inflammation.
COVID-19 – The disease caused by the virus, SARS-CoV-2.
Cross-reactive – Parts of the immune system that target one specific pathogen but may also react to similar pathogens. For example, a T cell* that targets the SARS-CoV-2 virus and might also react to a seasonal coronavirus if its structure is similar enough.
Cytokine – A small protein produced by immune cells, used to signal for a range of actions within the immune system, including inflammation. They are crucial for fighting infections, however they can begin to function incorrectly and cause damage (see cytokine storm).
Cytokine storm – An overproduction of immune cells and their activating compounds, cytokines. This is often associated with a surge of activated immune cells into the lungs, leading to lung inflammation, a build-up of fluids and respiratory distress.
D
Delta variant - The B.1.617.2 variant of SARS-CoV-2, formerly known as the 'Indian' variant as this is where it was first identified. The World Health Organisation recently announced a new naming system for variants of concern and variants of interest using the Greek alphabet, with names that are easy to pronounce and non-stigmatising.
Dendritic cell – Immune cells that are responsible for starting the adaptive immune response. They process antigen material, for example taken from pathogens, and then present those antigens on their cell surface for other cells such as T cells or B cells to see. They act as messengers between the innate and the adaptive immune systems.
E
Epitope – Part of an antigen that is recognised by the immune system, for example by antibodies, B cells, or T cells.
G
Gamma variant - The P.1 variant of SARS-CoV-2, formerly known as the 'Brazil' variant as this was where it was first identified. The World Health Organisation recently announced a new naming system for variants of concern and variants of interest using the Greek alphabet, with names that are easy to pronounce and non-stigmatising.
H
Homogeneous – Of the same kind; alike. Made of the same parts or elements or uniform in nature. For example, two people with the same kinds of immune responses to a virus have homologous responses.
Heterogeneous – Varied or diverse in nature, or of many different natures. For example, different kinds of immune responses.
I
IgA – Immunoglobulin A is an antibody that plays an important role in the immune function of mucosal membranes, such as in the nose, mouth, and lungs.
IgG – Immunoglobulin G is the most common form of antibody found in the blood. They are made by plasma B cells. Antibody tests for COVID-19 detect IgG and IgM antibodies, as they are a sign that someone has been exposed to SARS-CoV-2 in the past.
IgM – Immunoglobulin M is the largest type of antibody and the first to appear in response to a new antigen. It is made by B cells. Antibody tests for COVID-19 detect IgM and IgG antibodies, as they are a sign that someone has been exposed to SARS-CoV-2 in the past.
Immune memory – After your immune system is exposed to a pathogen, it may retain a memory of it. This means it can quickly launch an effective response if it comes across the same pathogen again. This is the mechanism that can provide some immunity to a pathogen for some time after first exposure. However, this depends on the specific pathogen as immune memory can vary both in how effective it is and how long it lasts for.
Immune evasion – The ways in which a pathogen escapes or avoids the defensive mechanisms of the immune system.
Immunity – the ability of the immune system to resist a pathogen, through mechanisms such as the innate and adaptive immune responses. We have varying levels of protection (and therefore immunity) depending on factors like how long it has been since we were first exposed to a particular pathogen, the overall health of our immune system, and whether that pathogen has mutated or changed since we were last exposed to it.
Immunocompromised – When someone’s immune system is not functioning normally or is impaired in some way, for example following chemotherapy for cancer or due to a genetic condition.
Immunopathology – The study of how immune responses can cause disease or harm to the body, for example looking at how an overactive inflammatory response can cause damage to tissues.
In vitro – Studies using samples outside their normal biological context, with in vitro meaning “in glass”. This includes studying cells in the lab. While there is a lot of merit and flexibility in doing experiments this way, the results do not always translate to what happens inside the body in the way that in vivo experiments can.
In vivo – Studies done within a living organism, i.e. studies carried out in animals or studies carried out in humans.. In vivo studies may have advantages over in vitro as they use living subjects, and are particularly useful when complex immune responses or medicines are involved.
Inflammation – A normal part of the protective immune response that attempts to rid the body of the cause of cell damage, as well as damaged cells and tissue. Inflammation can cause heat, redness, and swelling locally, as well as a fever. Sometimes, the inflammatory immune response can get out of control and be a problem, such as in a cytokine storm.
Innate immune response – One of the two main ways that the immune system fights off infection from pathogens, along with the adaptive immune response. The innate immune response involves a wide range of cells, molecules and mechanisms. It is broad and not specific to a particular bacteria or virus, and takes effect quickly after initial exposure. It can also trigger the adaptive immune response.
Interferons – IFN for short, a group of proteins released by cells in response to the presence of viruses. They are a type of cytokine, and signal to other cells that a protective immune response is needed. There are generally divided into Type I, Type II, or Type III IFN.
Interleukins – IL for short, a type of protein that is made by many different immune cells. They are essential for normal immune system function, and there are more than 50 types and related proteins (e.g. IL-1, IL-17). Their specific role depends on the type of interleukin, but it can include promoting inflammation, stimulating T cells, and antibody secretion.
L
Leukocytes – Also called white blood cells, these cells are part of the immune system that protect the body against pathogens. They are produced in the bone marrow and are found throughout the body. There are many types of leukocytes, including lymphocytes, neutrophils and monocytes.
Lymphocyte – A type of white blood cell that is made in the bone marrow. There are many subtypes including T cells, B cells and natural killer cells.
M
Machine learning – The use of a computer programme that improves automatically through experience and use of data. For example, an immunologist might use machine learning to find patterns in the number of different cell types in the blood sample of a group of patients. This can be useful when working with large datasets.
Macrophage – A white blood cell that is specialised in detection and destruction of broken-down cells, foreign particles, microbes, or anything else that it does not identify as part of the healthy human body. They can also release cytokines that initiate inflammation and activate other cells in the immune system.
Molecular analysis – A molecular pathway is a series of actions among molecules in a cell that leads to a certain product or change in that cell. Looking at the immune response at this detailed molecular level, rather than changes in numbers of or types of cells, is called molecular analysis.
Monoclonal antibodies – antibodies that are made in the lab to treat disease. They are made by manufacturing multiple copies of one antibody that can only target one specific receptor e.g. a receptor on the outer surface of cells.
Monocyte – A white blood cell that can change into other cell types, including macrophages and dendritic cells. They are part of the innate immune response and can influence the adaptive immune response. Their main functions are removing pathogens and debris, producing cytokines, and helping antibodies to target pathogens more specifically.
Mutation – An alteration in the genetic code of an organism, for example a virus, that can cause changes to its structure or behaviour. Each time a virus replicates, there is a small chance that a mutation will occur, although the chance of this depends on the type of virus (e.g. influenza virus tends to mutate more quickly than SARS-CoV-2).
N
Natural killer cells – Also called NK cells, they are a type of white blood cell and part of the innate immune response. They do not need antibodies or other signals to react to infection, meaning they can respond quickly to virus-infected cells.
Neutralising antibodies – Antibodies that defend a cell from bacteria or viruses by neutralising any effect it could have had on the body, meaning that it is no longer infectious or can no longer cause harm. They are part of the adaptive immune response.
Neutrophil – A white blood cell that is an important part of the innate immune response. They are produced in the bone marrow and are one of the ‘first responders’ at an inflamed site.
P
Pathology – The study of the causes and effects of disease or injury.
Pathogen – Any organism that can cause disease, for example bacteria, viruses, or fungi.
PCR test – Polymerase chain reaction (PCR) involves quickly making many millions of copies of a DNA sample, allowing scientists to test it in more detail. There are many different versions of PCR; reverse transcription polymerase chain reaction (RT-PCR) is most commonly used to test for SARS-CoV-2. It is typically very sensitive and specific when testing for the presence of SARS-CoV-2 genetic material and is more likely than some other tests to detect the virus in someone without any obvious symptoms (asymptomatic).
Phagocytes – Cells that protect the body from pathogens by ingesting them. They are an essential part of the immune system and include many types of white blood cells including neutrophils, dendritic cells, and macrophages.
[Immune] phenotypes – The specific profile of a person’s immune system. While the same cell types and features are present in most people, an immune phenotype includes aspects like the levels and activity of specific immune cells that are present in one person.
[Immune] phenotypes – The specific profile of a person’s immune system. While the same cell types and features are present in most people, an immune phenotype includes aspects like the levels and activity of specific immune cells that are present in one person.
R
Receptor – A structure, often found on the outer surface of cells, that receives a signal and either causes a response or passes on that signal to another structure. In immunology, a substance (e.g. a cytokine) can bind to an immune receptor and cause a response in the immune system.
S
SARS-CoV-2 – Severe acute respiratory syndrome coronavirus 2; the name of the virus that causes COVID-19. It is a single-stranded RNA virus.
Seasonal coronaviruses – Other than the coronavirus, SARS-CoV-2, that causes COVID-19, there are four other coronaviruses that can infect humans (called human coronaviruses OC43, HKU1, 229E, and NL63). These generally cause mild cold-like symptoms. About 15% of common cold cases are caused by a coronavirus.
Sero-positive – A person that produces a positive result in a blood serum (hence ‘sero’) test, for example for the presence of a virus like SARS-CoV-2.
Sero-negative – A person that produces a negative result in a blood serum test for a virus.
Spike protein – Part of the structure of SARS-CoV-2 (i.e. the ‘spikes’ on the outside of the virus) that allows the virus to attach and enter cells. It attaches to the ACE2 receptor on the surface of cells in our body, which triggers the cell to absorb it and allow the virus’ genetic material to enter. Many COVID-19 vaccines were designed to target the Spike protein.
T
T cell – Also known as T lymphocytes, T cells are one of the most important white blood cells of the immune system. They play a central role in the adaptive immune response. There are many different types of T cells. “Killer” T cells can kill virus-infected cells and use cytokines to recruit other immune cells to respond. “Helper” T cells indirectly kill cells identified as foreign to the body by directing other parts of the immune system to respond.
Therapeutic target – A specific immune pathway, mechanism, or molecule, that researchers believe could be influenced to improve health outcomes. For example, if levels of a specific type of immune cell are linked to excessive inflammation, researchers may attempt to treat that inflammation by targeting that type of cell.
V
Variant of Interest (VOI) - A variant of SARS-CoV-2 with mutations that make it distinct from the original/wildtype SARS-CoV-2 or other variants, and that has been identified by the World Health Organisation.
Variant of Concern (VOC) - A variant of SARS-CoV-2 with mutations that make it distinct from the original/wildtype SARS-CoV-2 or other variants. The difference between a VOI and a VOC is that a VOC has been shown to be associated with one of a number of changes such as increased transmissibility or decreased effectiveness of treatments or vaccines.
Viral evolution – The gradual change in a virus over time by genetic mutation.
Viral replication – The process by which a virus increases in number. To do so, viruses need to invade a host cell, where they hijack the biological structures to produce many copies of itself.
Viral variant – A mutated form of a virus, identified as distinct from the original form of a virus. By their nature, they can vary in how they are different from the original virus, depending on the specific mutation that occurred. The variant may be less transmissible or cause less severe symptoms. However, it can also become more transmissible or cause higher rates of mortality or not affect patient outcomes.