Glossary

adaptive immune system

The adaptive immune defence refers to the tailoring of an immune response to the particular foreign invader. It involves differentiating self from non self and involves B cells and T cells (lymphocytes). A key feature of the adaptive immune system is memory. Repeat infections by the same virus are met immediately with a strong and specific response.

antigen

Originally defined as any molecule which the body recognised as ‘non-self’, and against which an antibody was produced. This definition was extended to include any molecule that the body could recognise as foreign. This includes the fragments of molecules that are recognised by T lymphocytes. In the broadest sense, it has always been known that the immune system can recognise self molecules, even if it does not usually react against them. Consequently, the widest definition of an antigen is a molecule that can be recognised by the immune system, of which there are conventional non-self antigens and self molecules or autoantigens.

anti-viral proteins

A group of proteins that are induced by interferon, which when activated, inhibit protein synthesis and viral replication.

chronic

One that continues to produce disease symptoms and tissue damage over many months or years; some chronic infections (e.g. malaria) are characterised by periods of remission and relapse but the pathogen is never completely eliminated from the body.

cytokines

short-lived, short-range signalling molecules primarily synthesised and secreted by leukocytes that affect the activity of other cells participating in an immune response.

glycoproteins

A protein with one or more covalently attached carbohydrate groups (usually short sugar chains). Addition of such groups to proteins, termed glycosylation, is a form of post-translational modification.

innate immune system

The elements of the immune system that are continuously active and that do not depend on immune recognition of antigens by lymphocytes. Innate immune responses do not improve with repeated encounters with the same antigen or pathogen.

interferons

Cytokines that interfere with viral replication by the induction of anti-viral proteins. There are 3 main types of interferon IFNα, IFNβ and IFNγ. IFNγ, produced by active T lymphocytes and NK cells has many additional effects in controlling immune responses and inflammation.

latent

One in which the pathogens persist in or on the host’s body, but without producing symptoms; during the latent period, the host may or may not be infectious (i.e. capable of transmitting the pathogens to others).

nucleocapsid

The core of a virus containing its genetic material (DNA or RNA), within a protein coat (capsid).

receptor-binding domain (RBD)

For Sars CoV2, this is the region of the spike protein that binds to the ACE2 receptor on a cell, as the first step in virus infection of the cell.

sterile immunity

The complete elimination by the host’s immune response of the pathogens responsible for an infectious disease (e.g. the influenza virus is eliminated from the body as the illness resolves).

toll-like receptors

A group of receptors, located on the plasma membrane or on intracellular vesicles, that recognise components of pathogens (PAMPs) and transduce signals for inflammation.

type-1 interferon (IFN)

A cytokine produced by many cell types that signals to other cells to inhibit replication of viruses.

viral envelope

A phospholipid membrane that surrounds the nucleocapsid of some groups of virus. It is derived from the plasma membrane of the virus-infected cell.

viral tropism

The tendency of a particular virus to target specific cells which it can infect and then replicate within.

antigen processing

The process by which antigen is presented to lymphocytes in a form they can recognise. Most CD4+ T cells must be presented with antigen on MHC class II molecules, while CD8+ CTL cells only recognise antigen on MHC class I molecules. Antigen must be processed into peptide fragments before it can associate with MHC molecules

apoptosis

Type of cell death where particular cell populations die in a reproducible manner in every individual. Because of its predictable nature, this form of death was believed to occur as the result of a death ‘programme’, and so was named programmed cell death. Well-known examples are the loss of the cells between the digits (e.g. during the development of fingers). In adult tissues, cell death usually balances cell division, ensuring that tissues and organs retain the same size and structure as old cells are replaced. Apoptosis is a normal response in cells with DNA damage, serving to protect the body from cancer.

B cells

One of two main types of lymphocyte (cf. T cells) which, when activated, synthesises and releases huge quantities of soluble antibodies.

IgA

class of antibody that is prevalent in mucous secretions, and protects against infections in the gut, respiratory tree and genitourinary tract.

IgG

The main antibody in blood and tissue fluid. It has a large number of functions, including neutralising many toxic molecules, preventing viruses from attaching to cells, allowing phagocytes to recognise and internalise pathogens, and protecting the fetus and newborn babies. (It is the only antibody class that can cross the placenta.)

IgM

A class of antibody that is the first to be produced in an immune response.

immunoglobulins

An alternative name for soluble antibodies present in serum and tissue fluids.

immunological memory

the ability of the adaptive immune system to make an improved immune response on repeated encounters with the same antigen or pathogen.

lymphatic system

the connected system of lymphoid organs and lymphatic ducts present throughout the body.

lymphocytes

A major population of leukocytes including T cells, B cells and NK cells.

lymphoid organs

encapsulated organs such as thymus, lymph nodes and spleen and tonsils which contain collections of lymphocytes and cells involved in development of immune reactions.

lymphoid tissues 

include both the encapsulated organs of the immune system (eg lymph nodes) and unencapsulated collections of lymphocytes found in mucosal tissues.

MHC molecules

A group of proteins involved in antigen presentation to T cells.

NK cells

A group of lymphocytes that have the intrinsic ability to recognise and destroy some virally infected cells and some tumour cells.

proteasome

an intracellular organelle that breaks down proteins into polypeptide fragments.

T cells

Lymphocytes that differentiate primarily in the thymus and are central to the control and development of immune responses. The principle subgroups are cytotoxic T cells (Tc) and T helper cells (Th).

Assay

A method for quantitating biological material – in this case antibodies.

ELISA

Enzyme linked immunosorbent assay, is a set of techniques used for detection and quantitation of antibodies or antigens.

incidence of infection

The number of new cases of an infection in a given number of people in a defined period of time.

notifiable diseases

Diseases that must by law be reported to health authorities.

S-antibodies

Antibodies against the SARS-CoV2 spike protein.

variants of concern’ (VOC)

Variants of the SARS CoV2 virus identified by the WHO as having any combination of these characteristics: increased rate of transmission; producing more serious disease; ability to evade treatments or immune responses produced by vaccines; not being identified by current test procedures.

incidence

The number of new cases of a disease arising in a given period, usually a year, expressed as a proportion of the population at risk (the incidence rate).

N-antibodies

antibodies against the SARS-CoV2 nucleocapsid.

plasma

The non-cellular fluid component of blood containing soluble molecules, including proteins.

prevalence

The proportion of the population with a particular infection or disease at a particular point in time, or during a given period.

seroconversion

The appearance of specific antibodies in the blood serum as a result of infection or vaccination.

serology

The study of antigens and antibodies in patients’ sera.

seroprevalence

The proportion of a population which have antibodies against a particular infection at a defined time point.

serum

(Plural, sera.) The part of the blood left behind after cells, platelets and fibrinogen have all been removed, usually by clotting.

Basic reproduction number (R₀)

The average number of individuals directly infected by a single typical infective if the population were totally susceptible.