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An overreaction of the immune system to a normally harmless antigen, for example, pollen


An antigen which causes an allergic reaction.


Y-shaped proteins produced by plasma cells, which are specific for a particular antigen. The shape of the antigen-binding site of the antibody determines the target antigen it can bind to.


A chemical, such as penicillin, which prevents the growth of or destroys bacteria.


A particle that is recognised by cells of the immune system.

Antigen binding

The antigen-binding site of an antibody is located at the top of each of the two outstretched arms of the Y shape, where protein loops match the ones on specific antigens. This section of the antibody therefore determines which antigen the antibody binds.

Antigen presentation

Antigen presentation is a process in the body's immune system by which macrophages, dendritic cells and other cell types capture antigens, then processing them and present them on their cell surface on a specialised receptor. T cells specific for the presented antigen recognise it and become activated. This enables T cells to participate in the immune response

Antigen receptors

Receptors found on the surface of B and T cells which bind to specific antigens.

Antigen specificity

The ability of cells of immune system to specifically recognize a unique antigen.

Autoimmune disease

Autoimmune disease occurs when the immune system mistakes the body's own antigens as foreign and mounts an immune response against them. These responses can occur in many different parts of the body.


B cells

Lymphocytes that mature in the bone marrow. B cells are involved in the antibody response as they mature to form antibody producing plasma cells.


Cell-mediated response

The immune responses mediated by T cells, which can either bind directly to antigen-carrying cells and destroy them, or release a battery of cytokines to initiate the humoral immune response.

Cell surface receptors

Proteins within a cells membrane that allow communication between cells


The movement or migration of a cell in response to a chemical cue such as a chemokine (a signalling protein).

Circulatory system

The system of vessels through which blood circulates, transporting nutrients and oxygen to, and carbon dioxide and other waste materials from, cells all around the body.


Protein molecules secreted from cells as ‘messengers’ that relay signals to other cells.



A molecule which catalyses or accelerates a chemical reaction

Epithelial tissue

Tissues such as the skin, which line the cavities and surfaces of the body.


Germinal centres

A structure that forms within secondary lymphoid tissues such as the spleen and the lymph nodes that produces long-lived antibody-secreting plasma cells and memory B cells. In the germinal centre activated B cells can modify their antigen receptors, resulting in increased affinity for a specific antigen.


A type of white blood cell which have granules in their cytoplasm, containing chemicals such as reactive oxygen species and lytic enzymes which are released to kill pathogens.


Haematopoietic stem cells

Stem cells which give rise to all blood cell types.

Humoral response

A mechanism of pathogen elimination that is triggered by direct binding of antibodies to antigens.


Immune deficiency

A state in which the immune system's function is compromised resulting in lack of protection against infection.

Immunological memory

The ability of the immune system to remember a pathogen that has been encountered previously, allowing it to respond more rapidly and effectively to reinfection.


A localised area that becomes swollen, red, hot and often painful in response to injury or infection. This results from dilation of blood vessels in the area, which promotes access of immune cells to the site of inflammation.

Innate immune response

A rapidly activated line of defence that is first to respond to any pathogen that invades the body. The cells of the innate system do not recognise specific antigens, rather recognise distinctive molecular motifs such as sugars or nucleic acids that indicate the presence of a pathogen.


A member of a family of glycoproteins that are produced in response to some pathogens which enhance and modify the immune response.



Self contained vesicles containing digestive enzymes within a cell.


Lymph (from the Latin lymphia, or water) is the fluid that circulates through the lymphatic system. It is comprised of fluid that bathes tissues, and is transported through lymphatic vessels, via lymph nodes before mixing back with the blood. The lymph contains lymphocytes and may also carry bacteria and viruses from the tissues into lymph nodes where they will encounter cells of the immune system.

Lymph nodes

Small organs connected to the lymphatic vessels, which are found in multiple sites throughout the body. Lymphocytes circulate through the lymph nodes searching for foreign antigens that have been transported to the lymph nodes by dendritic cells.

Lymphatic system

The system of vessels through which lymph is transported from tissues back though lymph nodes before it mixes back with the blood.


Small white blood cells, comprising of three cell types: natural killer cells, B cells and T cells.


Enzymes contained in saliva, sweat and tears that have anti-bacterial properties.



A white blood cell which engulfs dead cells, cellular debris, foreign substances, microbes, and cancer cells by a process called phagocytosis.

Memory cells

Once an infection has been cleared, most of the B and T cells die off. However, a few remain in the body as memory cells, specific for the pathogen they previously encountered. This population of cells is long-lived and slow to divide under normal circumstances, but can expand very rapidly when the same pathogen is encountered again.

Messenger RNA (mRNA)

Nucleic acids that convey genetic information from DNA to enable it to be turned into protein. Proteins in cells, ribosomes, use mRNA as instructions to generate a specific sequence of amino acids that is then folded to become a fully functional protein.



A phagocytic white blood cell which is one of the first cells to respond to an infection, where it can engulf invaders.



The process by which a pathogen is marked by antibodies as foreign to induce phagocytosis and destruction by phagocytic white blood cells.



An organism that causes disease or illness to its host.


Cells such as macrophages and neutrophils which can engulf and subsequently digest bacteria, or fragments of dead or dying cells.


A vacuole within a phagocytic cell which contains an engulfed particle within a cell membrane.


One of the building blocks of body tissue, made of amino acids linked together into a specific three dimensional conformation.


Formed at the site of an inflammation, pus contains dead white blood cells and dead bacteria.


Reactive oxygen species

Chemically reactive molecules containing oxygen, for example oxygen ions and peroxides, which are released by neutrophils to destroy bacteria.



A constituent of the body’s normal tissue which could be recognised by the immune system, but is tolerated under healthy conditions.


The spleen is the largest lymphatic organ, through which the blood is constantly filtered to detect pathogens. The spleen also breaks down old red blood cells and stores platelets.


T cells

Lymphocytes that mature in the thymus.


A specialised organ in the chest where T cell development occurs.

Transcription factors

Proteins that bind to DNA and control the amount of DNA that is used as a template to create RNA.



A widening of the blood vessels.


A vaccine is used to generate a memory immune response without actually being infected by a pathogen. Vaccination creates immunological memory that enables a rapid and effective immune response when the pathogen is encountered later in life. A vaccine could be a solution of dead pathogens, pathogens modified to be less virulent, or parts of pathogens.


A small infectious agent that replicates only inside the living cells of other organisms.