Pathogens, disease and immunity definitions

Pathogens and non-specific defences

Pathogen

A biological agent that causes disease in a host, including cellular organisms such as bacteria, fungi and protoctists and acellular agents such as viruses.

Antigen

A molecule, usually a surface protein or glycoprotein, that is recognised as self or non-self and can trigger an immune response.

Non-specific defence

A defence that acts against any pathogen, without recognising which one it is, and acts at the same speed every time.

Exotoxin

A toxic protein secreted by a living bacterium, acting on a specific target in the host; Staphylococcus species are the standard example.

Endotoxin

The lipopolysaccharide of the outer membrane of a Gram-negative bacterium, released mainly when the bacterium dies; it causes fever and inflammation. Salmonella species are the standard example.

Toxoid

An exotoxin treated so that it is no longer toxic but keeps its antigens, used as a vaccine against diseases such as tetanus and diphtheria.

Stem rust

A disease of wheat and other cereals caused by the fungus Puccinia graminis, spread by wind-borne spores and entering through stomata.

Haustorium

A branch of a fungal hypha that pushes into a living host plant cell, inside a fold of the host's cell-surface membrane, and absorbs nutrients from it.

Interferon

A protein secreted by a virus-infected cell that binds to receptors on neighbouring cells and makes them resistant to viral replication, by inhibiting protein synthesis and destroying viral RNA.

Phagocyte

A white blood cell, such as a neutrophil or a macrophage, that engulfs and digests pathogens by phagocytosis.

Phagocytosis

The engulfing and hydrolysis of a pathogen or other foreign material by a phagocyte, forming a phagosome that fuses with a lysosome.

Phagosome

The vesicle formed when a phagocyte's cell-surface membrane closes around an engulfed pathogen.

Lysosome

An organelle containing hydrolytic enzymes, including lysozymes, kept separate from the cytoplasm.

Antigen-presenting cell

A cell displaying antigens from a pathogen or other non-self material on its own cell-surface membrane, such as a phagocyte after phagocytosis, an infected body cell or a B cell that has taken in the antigen.

The specific immune response

Self

Antigens present on an organism's own cells, which its lymphocytes do not respond to.

Non-self

Antigens not present on the organism's own cells: pathogens, transplanted tissue, cancerous cells with altered surface proteins, and toxins.

Lymphocyte

A white blood cell carrying receptors of a single shape, produced in bone marrow; T cells mature in the thymus and B cells in the bone marrow.

Clonal selection

The binding of an antigen to the lymphocytes whose receptors are complementary to it, activating those cells and no others.

Clonal expansion

The repeated division by mitosis of the selected lymphocyte, producing a large clone of genetically identical cells.

Cytokine

A small signalling protein secreted by one cell that binds to receptors on another and changes its behaviour; T-helper cells release cytokines to activate B cells, T-killer cells and phagocytes.

Interleukin

A cytokine released by one white blood cell to act on others, such as the interleukins an activated T-helper cell releases to stimulate B cells and T-killer cells to divide.

T-helper cell

A T lymphocyte carrying CD4 that binds antigen presented by another cell and, once activated, releases cytokines that activate B cells, T-killer cells and phagocytes.

T-killer cell

A cytotoxic T lymphocyte carrying CD8 that binds to an infected body cell displaying a complementary antigen and releases perforin, which makes pores in its membrane through which enzymes enter and trigger apoptosis.

B cell

A B lymphocyte: a white blood cell that matures in the bone marrow and carries antibody of one shape on its surface. When activated it divides to form plasma cells and memory cells.

Plasma cell

A differentiated B cell that secretes large quantities of a single antibody into blood and lymph. Most survive only a few days; a minority settle in the bone marrow and secrete for years.

Memory cell

A long-lived B or T lymphocyte produced by clonal expansion. It secretes no antibody, but divides rapidly if the same antigen is met again: a memory B cell into plasma cells, a memory T cell into T-helper or T-killer cells of its own type.

Antibodies, vaccination and the four kinds of immunity

Antibody

A protein with a quaternary structure of four polypeptide chains, produced by plasma cells, with variable regions forming two binding sites complementary to one antigen.

Variable region

The part of an antibody whose amino acid sequence differs between antibodies, forming the antigen-binding site.

Constant region

The part of an antibody that is the same in all antibodies of a class, and that binds to receptors on phagocytes.

Antigen-antibody complex

The structure formed when an antigen binds to the complementary binding site of an antibody.

Active immunity

Immunity produced when the individual's own lymphocytes respond to an antigen and form memory cells, after infection or vaccination. It develops slowly and lasts.

Passive immunity

Immunity from antibodies introduced from outside the body, across the placenta, in breast milk or by injection. It acts at once but fades, because no memory cells are made.

Vaccine

A preparation containing antigens from a pathogen, given to produce a primary immune response and memory cells without causing the disease.

Herd immunity

The protection of unvaccinated individuals that results when a large enough proportion of a population is immune for transmission not to be sustained.

Antigenic variation

Change in the antigens on a pathogen's surface, so that memory cells and antibodies produced against the previous form are no longer complementary to it.

Monoclonal antibody

A population of identical antibody molecules secreted by a clone of identical cells, and therefore all specific to the same antigen.

Hybridoma

A cell made by fusing an antibody-producing B lymphocyte from the spleen of an immunised mouse with a myeloma cell, so that it both makes the wanted antibody and divides indefinitely in culture.

Myeloma cell

A cancer cell of the bone marrow that divides indefinitely in culture; the lines used for fusion make no antibody of their own.

Placebo

An inactive substance made to look and be taken exactly like the drug being tested, given to the control group of a trial.

Double-blind trial

A trial in which neither the patients nor the doctors and researchers assessing them know who is receiving the drug and who the placebo or comparison treatment.

Phase I trial

The first test of a drug in humans, on a small number of healthy volunteers, to find whether it is safe, what dose is tolerated and how the body handles it.

Phase II trial

A test of a drug on a few hundred patients with the condition, to find whether it works, at what dose, and its side effects.

Phase III trial

A large randomised, usually double-blind trial on thousands of patients, comparing the drug with a placebo or the best existing treatment.

Infectious disease: transmission and control

Transmission

The passage of a pathogen from an infected host to an uninfected one, directly or through an intermediate such as water, food or a vector.

Vector

An organism that carries a pathogen between hosts without suffering the disease itself; the female Anopheles mosquito is the vector of malaria.

Carrier (of infection)

An infected person with no symptoms who still sheds the pathogen and can pass it on.

Endemic

Describes a disease permanently established in a region or population, as malaria is across much of the tropics.

Evolutionary race

The continuing coevolution of a pathogen and its host, in which better host defences select for better pathogen evasion, and the reverse.

Evasion mechanism

A feature of a pathogen that lets it avoid or survive the host's immune response, such as antigenic variation or surviving inside a phagocyte.

Cluster-randomised trial

A trial in which whole groups, such as villages, are allocated at random to the new method or the comparison, used where a method acts on a community.

Peer review

The checking of a scientific paper's methods, results and conclusions by other scientists in the same field before it is published.

Mass drug administration

Giving a drug to everyone in an area, whether or not they are ill, to reduce transmission.

Plant disease and plant defences

Callose

A polysaccharide of beta-1,3 linked glucose, deposited within minutes of infection at plasmodesmata, sieve plates and wounds, sealing the routes a pathogen would spread by.

Lignin

The tough, waterproof polymer of woody tissue, added to walls around an infection site over days to make the seal permanent.

Alkaloid

A bitter, nitrogen-containing compound made by a plant, toxic to or deterrent to herbivores; caffeine, nicotine and morphine are alkaloids.

Tannin

A phenolic compound in leaves and bark that binds proteins, inactivating digestive enzymes and making the tissue bitter and hard to digest.

Pheromone

A chemical signal released by an organism that changes the behaviour of another; used loosely in plant biology for the volatile signals plants release when attacked.