Key Words

It is important that you know the meanings of all the key scientific words as this will make it easier for you to understand what quesitons are asking and you may be tested on the meanings of some of these.

Look at the words below. Do you know a meaning for each? Click/Tap on the word to check that you know the correct definition.



When antibodies cause pathogens to clump together which makes it easier for phagocytes to destroy them and harder for the pathogen to invade.

Antibacterial compounds

These are a family of plant defensive chemicals which act on bacteria and include antibiotics, antiseptics and lysosomes which contain enzymes that break down bacterial cell walls.


Y shaped glycoproteins also called immunoglobulins which are made up of 2 short light chains and 2 long heavy chains. Each antibody has a constant region which is the same on every antibody and a variable region which is specific for one particular antigen and bind by the lock and key mechanism.

Antibiotic Resistant Bacteria

A bacteria which has mutated and is now resistant to antibiotics. Examples include MRSA and C Difficile.

Antifungal compounds

These are a family of plant defensive chemicals which act on fungal compounds including chitinases which break down the fungal cell walls.

Antigen-antibody complex

Formed when an antibody binds to a specific antigen.

Antigen Presenting Cell

A cell which presents antigens from pathogens on its own plasma membrane.


When an antibody binds to a toxin produced by a pathogen making the chemical harmless.

Artificial Active Immunity

When a person is given a vaccine which stimulates their body to make memory cells so that if they encounter the live pathogen they will be able to destroy it quickly.

Artificial Passive Immunity

When a person is given antibodies as their life is in danger which causes their body's phagocytes to fight off the infection.

Athlete's foot

A human fungal disease caused by Tinia Pedia that grows and digests the skin between the toes which is warm and moist. This causes the area to become itchy and sore.

Autoimmune disease

Where the body’s lymphocytes recognise the body’s own antigens and start destroying body tissue. Examples include – Type 1 Diabetes, Rheumatoid arthritis and Lupus.

B Effector cell

These cells divide rapidly to form a clone of plasma cells.

B Memory cell

These are cells which are matured in the bone marrow and live for a very long time after an infection. If they come into contact with a specific pathogen they will divide rapidly to make a clone of cells which will rapidly fight off the infection.


An example of a pathogen which has a prokaryotic cell and can be treated with antibiotics.

Bacterial meningitis

A bacterial infection of the meninges (the membranes around the brain). This can spread to other parts of the body and cause blood poisoning. Symptoms include a blotchy red/purple rash and a phobia of light. It affects the very young and teenagers aged between 15 and 19.

Black Sigatoka

A fungal infection which attacks and destroys the leaves of banana plants turning them black and reducing the yield by 50%. Fungicide can be used to treat this disease.

Blood clot

Caused by platelets, this creates a physical barrier to seal a wound.

Bone marrow

Where B Lymphocytes mature.


A polysaccharide produced by plants which contain β1,3 and β 1,6 linkages. It is synthesised quickly after the initial attack and deposited in cell wall, blocks sieve plates and is deposited in plasmodesmata.

Cell Mediated Immunity

This is the immune response which is used when a body’s own cells have been changed for example by infection with a virus, mutation or the early stages of cancer.

Climate change

The gradual increase in the Earth's temperature which can increase the spread of pathogens as organisms are able to survive in different areas (eg a mosquito can survive in wider areas). In plants this can cause increased rainfall and wind which also increases the spread of plant pathogens.

Clonal Expansion

Following the selection and activation of a lymphocyte with the specific receptor for the antigen causing the immune response, many copies of the lymphocyte are made through mitotic divisions.


The first milk produced by a mammalian mother for her new born, this is rich in antibodies.

Communicable disease

A disease that can be passed from one person to another and are caused by pathogens.


These are rituals in which can increase the spread of pathogens.


Chemicals which attract phagocytes to an area of infection as well as stimulating the hypothalamus to increase body temperature in the case of an infection. These are produced by mast cells as well as phagocytes which are dealing with an infection.

Direct contact

When a pathogen is transferred from touching the pathogen. In animals, examples include kissing, exchange of body fluids, skin to skin contact, or contamination such as having blood or faeces on from someone with the pathogen on your own skin. In plants, examples include touching a diseased plant.

Direct Transmission

Where a pathogen is transferred directly from one individual to another by a variety of mechanisms such as direct contact, inoculation, ingestion.

Droplet infection

Where droplets of water containing a pathogen are inhaled such as breathing in part of another person’s sneeze.


Where a communicable disease spreads through a local or national area.

Expulsive Reflex

Coughs, sneezes, vomiting and diarrhoea are all examples of these which help to push pathogens quickly out of the body.


When the hypothalamus is stimulated by cytokines causing an increase in temperature (above 37°c) which makes it more difficult for pathogens to reproduce and increases the chance of a phagocyte and a pathogen meeting (as they both have more kinetic energy).


These are more likely to cause infections in plants than in animals. They are eukaryotic organisms which are often multicellular and reproduce using spores.


This chemical makes blood vessels dilate and become leaky. This increases the temperature of the site of inflammation (which reduces pathogen reproduction), making it red and causes the area to swell as more tissue fluid is formed.


Human immunodeficiency virus which leads to Acquired immunodeficiency syndrome. This viral infection targets the immune system and can be spread by the mixing of body fluids through unprotected sex, sharing of needles, contaminated blood products and from mothers to babies during pregnancy and breast feeding.

Humoral Immunity

The immune system used to respond to antigens found outside of cells such as when bacteria and fungi invade.

Immunological memory

This is made up of memory cells which circulate the blood and cause a very fast response if a second infection occurs of the same pathogen.

Indirect Transmission

Where a pathogen is transferred indirectly via a vector such as another organism.

Inflammatory Response

A localised response to a pathogen or irritant which results in pain, heat, redness and swelling of the tissue also known as inflammation.


A viral infection of the ciliated epithelial cells of the respiratory system which destroys the cells and leaves the airways open to secondary infections. It can particularly affect the very old or the very young as well as pigs, birds and chickens.


When contaminated food or drink containing the pathogen is transferred into the body.


Where a pathogen enters the body such as through sex or from an animal bite or a puncture wound to the skin in sharing needles or cuts.


Another example of a plant defensive chemical, in this case they are toxic to insects.

Insect repellents

This an example of a plant defensive chemical and examples include pine resin and citronella from lemon grass. These repel insects that may want to eat the plant an introduce pathogens.


Enzymes which destroy bacterial and fungal cell walls. As well as being found as part of mucus, they are also found in the urine, tears and stomach acid to prevent entry of pathogens into the body.

Major histocompatibility complex

Special glycoproteins inside macrophages which move the antigen from a pathogen into the plasma membrane of a phagocyte.


Spread by the bites of mosquitoes and caused by the protoctista Plasmodium. This parasites life cycle involves two hosts and is an example of indirect transmission. The pathogen invades red blood cells, liver and brain cells.

Mast cells

These are cells which are activated during inflammation and release histamines and cytokines.

Mucous membranes

Tissues that produce a sticky mucus which contains lysozymes as well as phagocytes and are found lining many body tracts to prevent pathogens entry to the body.

Natural active immunity

The immunity that you are left with following an infection and illness from a pathogenic disease. This immunity prevents you from getting the same illness twice.

Natural passive immunity

The immunity found in new born baby’s due to antibodies that are passed from the mother across the placenta an in the breast milk.

Non-communicable disease

A disease that cannot be passed on from one person to another such as heart disease, most types of cancer and most disorders of the nervous, endocrine and digestive systems.


Chemicals that bind to pathogens to make them more easily recognisable to phagocytes. There are lots of different types but antibodies are the most common example.


A factor which can increase the chance of a pathogen being communicated from person to person due to the close proximity to others. This can also affect plants which are planted too closely to each other.


Where a communicable disease spreads through several countries or continents.


An infective organism that can pass on a communicable disease. These include bacteria, viruses, fungi and protoctista.


A chemical which is used to destroy cells by making a hole in the side of the plasma membrane and cause the chemicals to leak out.


Specialised white blood cells which engulf and destroy pathogens by phagocytosis. There are two types of phagocytesneutrophils and macrophages.


The vacuole inside a phagocyte which contains the pathogen and has a lysosome attached to it.


The vacuole inside a phagocyte which contains the pathogen.


The use of genetic screening in drug treatment so that the most effective drugs are used to treat an illness based on the genes that the patient has.

Plasma cells

Matured in the bone marrow, these are cells which produce antibodies to a specific antigen. These cells only live for a few days producing 2000 antibodies per second.

Potato Blight

Caused by a protoctista which destroy leaves and tubers and can cause damage to crop yields. Other varieties exist which can infect tomatoes and late blight which affect other crops.


These are eukaryotic single celled organisms such as malaria, sleeping sickness and dysentery.

Ring rot

A bacterial disease of potatoes, tomatoes and aubergines which damages leaves, tubers and fruit. Once a field has been damaged by this bacteria it cannot be reused for at least 2 years.

Ring worm

A fungal disease which causes a white/grey crusty circular areas of skin. It affects cattle, dogs, cats and humans.


A chemical (neurotransmitter) which is released following a cut and makes the smooth muscle walls of blood vessels contract so they reduce the blood supply to an area to prevent blood loss.


A physical barrier which prevents entry of pathogens into the body. It is covered in a flora of healthy microorganisms which prevent pathogens from having space to grow as well as producing sebum an oily substance which prevents pathogen growth.

Socioeconomic factors

A lack of trained health workers or a lack of public information which can lead to a pathogen being able to spread more easily.


A pathogen which is able to survive unhospitable conditions – these are usually bacteria which can survive harsh cold or extreme hot environments as well as water. It is a way that pathogens such as the black sigatoka can indirectly infect other plants.

Synthetic Biology

The use of bacteria to produce rare or expensive drugs.

T Helper cells

These have CD4 receptors on their plasma membrane which can bind to the antigens on antigen presenting cells causing the T Helper cell to be activated. The activation causes interleukins to be produced which stimulate B Lymphocytes and other types of T cells as well as stimulating macrophages to engulf more of the pathogens.

T killer cells

These destroy pathogens which carry a specific antigen that they are made for. They do this by producing a chemical called perforin which makes a hole in the side of the plasma membrane causing the cytoplasm to leak out.

T memory cells

These are cells which form part of the immunological memory by circulating in the blood for a long time after an infection. If they meet the pathogen again they divide rapidly to produce a large number of clones of T killer cells and much faster response to the infection.

T regulator cells

These are cells which supress the immune system after a pathogen has been eliminated and are also involved in stopping autoimmune diseases.


An enzyme which triggers a cascade of reactions resulting in the formation of a blood clot.

Thymus Gland

Where T Lymphocytes mature.

Tobacco Mosaic Virus

A pathogen which infects tobacco plants along with many other types of plants. This pathogen restricts growth and although resistant crops exist there is no cure.


A chemical produced by a pathogen which can cause damage to cell membranes, inhibit enzymes or interfere with the genetic functions of the cell.


A bacterial disease which infects animals including humans, cows, pigs, badgers and deer. It damages lung tissue as well as suppressing the immune system.


A weakened or dead form of a pathogen which can be used to simulate the immune system.


A living or non-living factor that transmits a disease from one place to another. Common examples include wind, water, insects, animals such as fleas and rats which can transmit disease and spores which survive on clothing and machinery for many years.


A non-living pathogen which consist of DNA or RNA surrounded by a protein coat. They infect cells and cause them to make more copies of the pathogen.

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