6.4 Response to Infection

What is active immunity? (2)

- Active immunity develops when the body makes its own antibodies in response to battling a pathogen.

- It involves the production of memory B cells and is typically long-lasting.

What is passive immunity? (2)

- Passive immunity develops when a person is given antibodies that have come from another animal or person.

- It is often administered as an injection when an individual is infected and needs immediate protection.

What is the difference between natural and artificial immunity? (3)

- Natural immunity is immunity that develops after an infection.

- It results from the body's primary immune response to a particular antigen, which leads to the development of immunological memory.

- Artificial immunity results from the priming of the immune system through the use of vaccines.

What are the key differences between active and passive immunity? (3)

- Active immunity is acquired when a person's own immune system produces antibodies in response to an antigen.

- It is a long-lasting form of immunity that provides memory for future responses.

- Passive immunity is acquired through the transfer of pre-made antibodies from one individual to another, such as from a mother to her baby.

How is an inflammatory response triggered? (3)

- An inflammatory response is triggered when an invading pathogen has penetrated the body's initial lines of defence.

- When the tissue is damaged, mast cells and basophils located in the connective tissue become specialised.

- These specialised cells then release histamines, which initiate the inflammatory response.

How does histamine contribute to the inflammatory response? (3)

- Histamines cause blood vessels, particularly the arterioles, to dilate, which brings more blood to the infected tissue.

- They also cause the walls of the capillaries to become more leaky, allowing plasma fluid to escape.

- The smooth muscle of the arterioles relaxes, which further increases the blood flow to the infected area.

How does inflammation lead to the swelling of infected tissue? (3)

- In a process called diapedesis, the cells in the walls of the capillaries draw away from each other.

- This causes the capillaries to become leaky, leading to the formation of more tissue fluid than usual.

- The additional volume of tissue fluid, along with leucocytes and antibodies, stimulates the local swelling of the infected area.

How does a fever help the body fight infection? (3)

- An infection by a pathogen can cause the hypothalamus to reset the body to a higher physiological temperature.

- This raised body temperature reduces the ability of many pathogens to reproduce, making them easier to defeat.

- It also helps to create and maintain an optimum temperature for the enzymes involved in the inflammatory response to work efficiently.

What are three functions of complement proteins in the immune response? (3)

- Complement proteins help by attracting more phagocytes to the site of an infection.

- They can bind to and form pores in the surface membrane of foreign cells, causing them to lyse.

- They also bind to the surface membrane of foreign cells, which aids the process of opsonisation by phagocytes.

What is opsonisation? (2)

- Opsonisation is a process initiated by proteins called opsonins.

- The opsonins bind to the surface of bacteria, making them more recognisable to phagocytes for destruction.

What is the role of interferon in the immune response? (3)

- Interferon is a type of cytokine chemical that is released by cells which have been infected by a virus.

- Interferons trigger the release of antiviral proteins from neighbouring healthy cells.

- These antiviral proteins work by preventing the virus from reproducing.

What are two differences between neutrophils and macrophages? (2)

- Neutrophils are derived from granulocytes and can only ingest a small number of pathogens before dying.

- Macrophages are derived from agranulocytes and have a substantial capacity for ingesting pathogens.

How does a phagocyte destroy a pathogen after identifying it? (3)

- The phagocyte moves towards the pathogen and engulfs it, enclosing it within a vesicle called a phagosome.

- The phagosome then fuses with a lysosome, which is an organelle containing powerful digestive enzymes.

- The lysosome releases its enzymes into the phagosome, which break down the engulfed pathogen.

What is pus? (2)

- Phagocytes that have died after being involved in the immune response are known as pus.

- Pus is an accumulation of dead cells and is mainly composed of neutrophils.

What is the Major Histocompatibility Complex (MHC)? (3)

- It is a protein marker used to distinguish the body's own cells (self) from foreign cells (non-self).

- Most proteins enable the immune system to specifically attack foreign pathogens without attacking the body's own cells.

- MHC proteins are special proteins that enable the body's cells to present antigens to the immune system.

What are MHC class I molecules? (2)

- Class I molecules are found on all nucleated cells in the body.

- They work by binding to endogenous antigens and displaying them on the cell surface to identify the cell's "self" status.

What are MHC class II molecules? (2)

- Class II molecules are found on some immune system cells like macrophages, neutrophils, and dendritic cells.

- They are used by these antigen-presenting cells to stimulate an immune response against a particular invading pathogen.

What is the role of B-cells? (2)

- B-cells are short-lived cells produced during clonal expansion.

- Their main function is to produce large quantities of antibodies against a specific antigen, which are then secreted into the blood.

What is the role of memory B cells? (3)

- Memory B cells are a key component of the secondary immune response.

- Upon a second infection by the same pathogen, memory B cells rapidly differentiate into plasma cells.

- This allows for the quick production of antibodies, enabling the body to fight the infection before it can cause illness.

How are B cells activated and differentiated? (3)

- T-helper (Th2) cells activate phagocytosed B cells.

- Once activated, B cells will rapidly proliferate through a process known as clonal expansion.

- Following this expansion, the rapidly proliferating B cells differentiate into either plasma cells or B memory cells.

What are two key features of B cells? (3)

- B cells are a type of lymphocyte that are produced in the bone marrow.

- B cells are located in the lymph glands and can also be found free in the body.

- They have membrane-bound globular proteins on their cell surface that are capable of binding with antigens.

How do plasma B-cells and memory B-cells work together? (3)

- Plasma B cells are created in response to a primary infection and produce a large amount of antibodies.

- Memory B cells are also created during the primary response but are involved in the secondary response.

- Upon a second infection by the same pathogen, memory B cells rapidly differentiate into plasma B cells to quickly produce antibodies.

How does clonal selection in B-cells work? (3)

- B cells integrate some antibodies into their cell membranes, where the antibodies act as B cell receptors.

- The variable regions of these antibodies have a shape that is complementary to a specific, unique antigen.

- Contact between this antigen and the cell surface receptor activates the B cell, which then begins to divide.

What happens after a B-cell has been stimulated by an antigen? (3)

- The stimulated B-cell processes the antigen and presents it on its MHC-II complex.

- Helper T-cells produce cytokines which cause the B-cell to divide by mitosis.

- The B-cell divides to produce both B memory cells and B effector cells, which differentiate into plasma cells.

What is the difference between the origin and maturation sites of T and B cells? (2)

- T cells originate in the bone marrow and mature in the thymus gland.

- B cells both originate and mature in the bone marrow.

What is the difference between the roles of T cells and B cells? (2)

- T cells are responsible for recognising antigens and triggering an immune response.

- B cells produce reactive antibodies that neutralise the invading pathogen.

What is the role of Antigen Presenting Cells (APCs) in the immune response? (2)

- Antigen presenting cells (APCs) activate the adaptive immune response.

- They work by engulfing foreign pathogens and then presenting their antigens to other cells of the immune system.

How are cytotoxic T-cells activated? (3)

- Neutrophils and macrophages phagocytose pathogens and act as APCs.

- Interleukin is secreted from the APCs.

- T-helper cells are activated and then activate pathogen-specific cytotoxic T-cells and B-cells.

How do T-cells destroy virally infected cells? (3)

- The T-cell binds to the MHC molecules of an infected cell.

- It produces proteins called perforins, which pierce holes in the cell membrane of the target cell.

- It also produces granzyme enzymes, which enter the cell and induce apoptosis (programmed cell death).

What happens after a target cell is killed by a cytotoxic T-cell? (2)

- The T-cell is unaffected, detaches from the killed cell, and releases interferon proteins.

- These interferon proteins inhibit viral replication in the surrounding area, helping to protect nearby cells.

How does a cytotoxic T-cell initiate the destruction of an infected cell? (3)

- A cytotoxic T-cell binds to the MHC molecules present on the surface of an infected cell.

- It then produces proteins called perforins, which create holes in the cell surface membrane of the target cell.

- The T-cell also produces enzymes known as granzymes, which poison and induce apoptosis in the target cell.

What is clonal deletion? (3)

- Clonal deletion is the process of destroying any developing immune cells that respond to self-antigens.

- This process, known as central tolerance, ensures that the immune system does not attack the body's own tissues.

- In peripheral tolerance, clonal deletion removes mature T and B cells that respond to self-antigens in the body's periphery.

What is the role of interferons in the immune response? (2)

- Before a target cell undergoes apoptosis, it secretes interferon proteins.

- These proteins can activate natural killer (NK) cells in the surrounding area and enhance the anti-viral defence of other body cells.

What is the concept of 'self' and 'non-self' in immunology? (2)

- 'Self' refers to the body's own native cells and tissues, which the immune system should not attack.

- 'Non-self' refers to foreign bodies, such as pathogens or toxins, that the immune system should recognise and attack.

What are three types of white blood cells? (3)

- T-cells and B-cells (lymphocytes).

- Natural killer cells.

- Phagocytes, such as neutrophils and macrophages.

What is the role of T-cells in the immune system? (2)

- T-cells, also known as T-lymphocytes, help to identify and attack infected cells.

- Memory T-cells also exist, "remembering" the invader to respond quickly if it tries to infect the body again.

What are T-cells? (3)

- T-cells are a type of lymphocyte that is produced in the bone marrow.

- They then mature and become biologically active within the thymus gland.

- They are a crucial component of the cell-mediated adaptive immune system.

How does an antigen-presenting cell activate a T-cell? (2)

- T-cells have receptors on their outer membrane that can be specific to an antigen presented by an APC.

- The binding of the T-cell receptor to the antigen-presenting cell triggers the T-cell, activating it to mount an immune response.

What happens after a T cell is triggered by an antigen-presenting cell? (3)

- The triggered T cell is stimulated to reproduce, which forms a clone of identical T cells.

- These cloned T cells are then activated to become active T helper cells, which release cytokines to stimulate B cells.

- A proportion of the cloned cells is formed as inactive T memory cells, which are stored in the body.

How is a T killer cell activated and cloned? (3)

- A specific T killer cell binds to the corresponding antigen/MHC complex displayed on the surface of an antigen-presenting cell.

- Activated T helper cells then release chemicals called cytokines.

- Cytokines stimulate the T killer cell to divide and form a clone of identical cells.

How does a T killer cell initiate the destruction of an infected cell? (3)

- The active T killer cell binds to the infected body cell.

- It then releases enzymes that create pores in the membrane of the infected cell.

- The pores disrupt the fluidity of the surface membrane, which enables the free entry of water and ions, causing the cell to lyse.

What is the role of T memory cells? (2)

- During an initial immune response, some of the activated T-cells become T memory cells.

- These memory cells persist in the body, which allows for a more rapid and effective response if the same antigen invades again.

What are lymphocytes? (3)

- Lymphocytes are a type of white blood cell involved in the specific immune system.

- They originate in the bone marrow and are suspended in the lymph and blood.

- Lymphocytes possess receptors on their surface that allow them to bind to specific foreign antigens.

What is the main lymphocyte in the cell-mediated response and what does it do? (3)

- The cell-mediated response primarily involves T killer cells.

- They can respond to antigen-presenting cells.

- They can respond to and destroy cancer cells.

What is the role of macrophages in the primary immune response? (3)

- Macrophages are a type of phagocytic cell that are produced in the bone marrow.

- When a pathogen enters the body, chemicals are produced that attract macrophages to the site of infection.

- The macrophage engulfs and digests the pathogen, then presents its antigens on its surface to activate other immune cells.

What is phagocytosis? (1)

It is a process by which specialised white blood cells, called phagocytes, engulf and destroy pathogens.

How does a phagocyte recognise and engulf a pathogen? (3)

- A phagocyte detects a pathogen by recognising specific molecules on its surface.

- The phagocyte's receptors bind to the pathogen, sometimes aided by opsonins like antibodies.

- The phagocyte then extends its cytoplasm, known as pseudopodia, to surround and engulf the pathogen.

What happens to the products after a pathogen is digested by a phagocyte? (2)

- Some useful products may be absorbed into the cytoplasm of the phagocyte.

- Other products, specifically antigens, are loaded onto MHC class II molecules and presented on the cell surface.

How do phagocytes find pathogens? (2)

- Chemicals released by pathogens can act as a chemical beacon, attracting phagocytes.

- Phagocytes move towards the site of infection by following this chemical gradient, a process known as chemotaxis.

What are antigens? (2)

- Antigens are molecules, typically proteins, found on the surface of cells that can trigger an immune response.

- They are unique to particular cells and pathogens, allowing the immune system to identify them as foreign.

What is an antibody? (2)

- An antibody is a protein that is produced by the body in response to a specific antigen.

- The binding of an antigen-antibody complex activates gene expression to synthesise the correct corresponding antibodies.

What are three functions of an antigen-antibody complex? (3)

- Complement activation, which involves attracting phagocytes to the site of infection.

- Neutralisation, where antibodies neutralise the effects of bacterial toxins by binding to them and blocking their action.

- Lysis, which is the breaking open of bacterial cells, often facilitated by the action of other factors.

What is the basic structure of an antibody? (3)

- Antibodies are Y-shaped molecules composed of four polypeptide chains: two long heavy chains and two short light chains.

- The constant region has the same amino acid sequence in all antibodies and is used to bind to phagocytes.

- The variable region has a unique structure for each antibody and serves as the antigen-binding site.

How does the immune system produce so many different types of antibodies? (2)

- The immune system is able to produce a wide variety of antibodies, each with a unique structure.

- This is achieved by rearranging genetic material in a process called somatic hypermutation, which allows the immune system to rapidly generate new antibodies.

How does the structure of an antibody allow it to recognise and neutralise specific antigens? (3)

- The structure of an antibody includes an antigen-binding site, which is composed of the light and heavy polypeptide chains.

- This antigen-binding site has a shape that is specific and complementary to a particular antigen.

- An antibody attaches to an antigen and can neutralise it by several mechanisms, such as blocking its function, triggering its destruction, or marking it for removal

How can a vast variety of different antibodies be produced from a limited number of genes? (2)

- This is possible due to the post-transcriptional processing of the genes that code for antibody proteins.

- This process allows for the production of many different versions of mRNA, and therefore different versions of the final antibody proteins.

What are the antibody functions of agglutination and opsonisation? (2)

- In agglutination, antibodies bind to many pathogens, causing them to clump together, which prevents them from spreading and makes them easier for phagocytes to engulf.

- In opsonisation, the antibody acts as an opsonin, which is a chemical that makes pathogens more recognisable to phagocytes, enhancing phagocytosis.

What is the role of the hinge region in an antibody? (2)

- The hinge region is the area of flexibility in the antibody molecule.

- This flexibility allows the branches of the Y-shaped molecule to move, which allows them to bind to multiple antigens that may be spaced apart.

What are three functions of antibodies? (3)

- Neutralisation, where antibodies bind to and inactivate toxins or viral attachment proteins to prevent them from infecting host cells.

- Agglutination, where antibodies clump pathogens together, which stops them from spreading and makes them easier for phagocytes to engulf.

- Lysis, where enzymes can bind to antibodies on a pathogen's surface and catalyse reactions that break down the cell membrane.

What are monoclonal antibodies? (1)

Monoclonal antibodies are identical antibodies that are produced by a single clone of plasma cells in response to a specific antigen.

What are three functional roles of monoclonal antibodies? (3)

- Diagnostic procedures, such as identifying specific pathogens in patient samples.

- Cancer therapy, where they can be designed to be specific to tumour markers on cancer cells to neutralise them or deliver drugs.

- Medical diagnosis, where they are used to detect particular antigens, for example in pregnancy test kits.

What are the initial steps of the hybridoma method for producing monoclonal antibodies? (3)

- A mouse is injected with a specific antigen to stimulate an immune response where B cells produce corresponding antibodies.

- Spleen cells, containing the lymphocytes for antibody production, are removed from the animal.

- These spleen cells are then fused with tumour cells, known as myeloma cells, to form hybridoma cells.

What happens after hybridoma cells are created? (2)

- The individual hybridoma cells producing the desired antibody are isolated, cloned, and cultured on a large scale.

- The monoclonal antibodies produced by the hybridoma cells are then separated, purified, and collected for future use.

What is the difference between monoclonal and polyclonal antibodies? (2)

- Monoclonal antibodies have a highly specific antigen-binding site and can only bind to a single epitope of a particular antigenic marker.

- Polyclonal antibodies have a diverse antigen-binding site that can recognise and bind to several different epitopes of a marker.

What is the immune system? (2)

- It is a complex network of cells, tissues, and organs.

- It works to protect the body from harmful invaders like bacteria, viruses, and other parasites.

How does the immune system work? (3)

- It recognizes and attacks foreign invaders that could harm the body.

- White blood cells, also known as leukocytes, are responsible for detecting and eliminating these invaders.

- When a foreign invader is detected, the immune system produces specific proteins called antibodies to help fight the invader.

What are three characteristics of the specific immune response? (3)

- It has specificity, meaning it responds to particular foreign cells or antigenic markers.

- It is diverse and is able to recognize approximately 10 million different antigens.

- It has immunological memory, which allows it to form a rapid response to reinfection by the same pathogen.

What are the two main types of immunity? (2)

- Innate immunity is the body's non-specific, first line of defence against pathogens, which is present from birth.

- Acquired immunity is specific immunity that is developed after exposure to a pathogen, either naturally or through vaccination.

What is the primary immune response? (3)

- The primary immune response involves the production of antibodies by plasma cells in response to an initial encounter with an antigen.

- It also includes the activation of T killer cells to fight the specific pathogen.

- A key characteristic of this response is that there is a lag period before the immune response can effectively fight the invading pathogen.

What is the innate immune response? (2)

- It is the body's non-specific, first line of defence against pathogens.

- It is a rapid response that occurs immediately after a pathogen enters the body.

What are the components of the innate immune response? (2)

- The components include physical barriers like skin and mucous membranes.

- It also includes physiological and chemical mechanisms such as antimicrobial substances, inflammation, and fever.

Where are the cells of the immune system found? (2)

- Cells of the innate immune system can be found in most areas of the body, including just ahead of the body's mucous membranes and deep inside tissues.

- They are also often found in the lymphatic system and can travel through the body via its complex transport system.

How is the innate immune response initiated in the blood? (2)

- A response is initiated when macrophages recognise foreign antigens on the surface of a pathogen that has entered the blood.

- Macrophages in the blood and lymph nodes then carry out phagocytosis to destroy the pathogen.

What happens during the secondary immune response? (3)

- Phagocytes activate three types of T-cells: T-helper, Cytotoxic T, and T-memory cells.

- T-helper cells activate B cells and T-killer cells.

- Cytotoxic T cells then directly kill the pathogens.

How do memory cells contribute to the secondary immune response? (2)

- T-memory cells provide long-term immunity against a specific pathogen.

- B-memory cells rapidly differentiate to produce antibodies and plasma cells, which in turn produce more antibodies to break down pathogens.

What is the difference between the innate and adaptive immune responses? (3)

- The innate response is a nonspecific response that occurs immediately upon exposure to a pathogen.

- The adaptive response is a specific response that is tailored to a particular pathogen and takes longer to develop.

- The innate response provides a rapid initial defence, while the adaptive response provides long-term, specific protection.

How does a fever help the innate immune response? (2)

- A fever is a rise in body temperature that occurs in response to a pathogen.

- The elevated temperature helps to kill pathogens and reduces their ability to reproduce.

What is the role of physical barriers in the innate immune response? (3)

- Skin acts as a physical barrier to prevent most pathogens from entering the body.

- It is also covered in antimicrobial substances that help kill pathogens on contact.

- Mucous membranes line the body's openings and contain antimicrobial substances to kill pathogens before they can enter the body.

What is the adaptive immune response? (3)

- It is a specific line of defence mechanism that is activated upon exposure to foreign substances like pathogens.

- It is composed of B-cells and T-cells, which produce a tailored reaction to combat the invading agent.

- It is essential for the survival of living organisms and plays a crucial role in preventing recurrent infections by providing immunological memory.

How does the adaptive immune response work? (3)

- The response is initiated through the interaction of immune cells, such as T and B cells, with specific foreign substances known as antigens.

- Once an antigen is recognised, the immune system produces specific antibodies and activated immune cells to combat it.

- The body then develops a memory of the antigen, which allows for a faster and more effective response to future infections.

Why is the adaptive immune response important for preventing disease? (3)

- It is essential for preventing disease by recognising and neutralising harmful pathogens.

- The ability to produce specific antibodies provides a targeted defence against pathogens.

- The development of immunological memory allows for a faster and more effective response to future infections by the same invader.

How does the adaptive immune response remember previous exposures to specific antigens? (3)

- The adaptive immune response can remember previous exposures to specific antigens.

- This memory is achieved through the survival of specific immune cells, such as memory T and B cells, which can respond quickly to subsequent re-exposures.

- This allows for a faster and more effective immune response upon re-exposure to the same antigen, often preventing illness.

What are immunological memory and humoral immunity? (3)

- Immunological memory is the mechanism that provides organisms with long-term protection against pathogens they have previously encountered.

- Humoral immunity is a type of immunity that is maintained by secreted macromolecules, which circulate in the extracellular fluids, such as lymph and blood.

- Humoral immunity is primarily mediated by B-cells and the antibodies they produce.

What is the humoral response? (3)

- The humoral response is the part of the immune system that reacts to antigens from extracellular pathogens, such as bacteria.

- It results in the production of antibodies which are not attached to cells but are secreted into the body's fluids.

- The response includes the activation of B cells by T helper cells and an effector phase, where large quantities of antibodies are produced.

What is the difference between artificial active and artificial passive immunity? (3)

- Artificial active immunity involves vaccination, where a non-infective form of a pathogen is introduced into the body.

- The body then produces its own antibodies and memory cells, which results in long-term immunity.

- Artificial passive immunity occurs when pre-made antibodies that were formed in one individual are extracted and then injected into another for immediate, short-term protection.

How does immunological memory lead to a more effective secondary immune response? (3)

- During the primary response, B memory cells are produced, which provides the body with immunological memory against a specific pathogen.

- If the same pathogen enters the body for a second time, its antigens will be recognised by these memory cells.

- This stimulates the B memory cells to divide rapidly and produce a greater quantity of antibodies in a much shorter time interval.

What is the difference between natural active and natural passive immunity? (3)

- In natural active immunity, the body is stimulated to produce its own antibodies in response to an antigen that has been encountered naturally, such as through an infection.

- In natural passive immunity, an individual receives pre-made antibodies from another source, for example, a baby receiving antibodies from its mother.

- These antibodies can be passed via the placenta during pregnancy, or through breast milk after birth.

Why is herd immunity particularly important for infants? (2)

- Many infants are too young to receive particular vaccines, as most are given at specific intervals from two months onwards.

- Therefore, strong herd immunity in the population is necessary to protect infants and other vulnerable people from preventable diseases.

What happens in an autoimmune disorder? (2)

- Autoimmune disorders are conditions in which the body's own immune system mistakenly identifies healthy cells as foreign and attacks them.

- These diseases can be acute or chronic, with examples including Type 1 diabetes and rheumatoid arthritis.

Why can organ transplants be rejected? (3)

- The immune system of the recipient may recognise the donor organ as foreign and attack it.

- MHC genes vary between individuals, so if the donor organ's cells are recognized as non-self, the immune system will attack them.

- If the T cells in the recipient recognise the donor cells as foreign, they can trigger a powerful immune response against the transplanted organ.

What is vaccination? (2)

- A vaccination is a medical procedure that involves administering a weakened or inactive form of a pathogen, or a part of a pathogen, to a person.

- This is done in order to trigger the production of antibodies and memory cells, which helps to protect the person from future infections by the same pathogen.

How do vaccinations work? (3)

- A vaccination involves administering a weakened or dead form of a pathogen, or a piece of the pathogen, to a person.

- This stimulates the body into thinking that it is being invaded by a real pathogen, triggering a primary immune response.

- This process of 'tricking' the body into producing its own protection is known as active immunity and helps to protect against future infections from the same pathogen.

What are three benefits of vaccination? (3)

- It protects individuals from contracting specific diseases and infections.

- It helps to reduce the spread of diseases and infections within communities.

- It helps in the prevention of large-scale outbreaks and epidemics.

What are two risks associated with vaccination? (2)

- Attenuated vaccines are often cultured in eggs, which can cause an allergic reaction in some people.

- It is possible for children to become ill after receiving a vaccination as their immune system is not yet fully developed.

How do vaccines work to provide immunity? (3)

- Vaccines are preparations of dead or weakened pathogens that can be safely injected into the body.

- These preparations contain the necessary antigens to stimulate the immune system to produce specific antibodies and memory cells.

- This allows the body to have an efficient mechanism in place to rapidly detect and fight the pathogen if it enters the body in the future.

How do vaccines lead to long-term immunity? (3)

- A vaccine introduces a type of antigen which stimulates the individual's immune system to produce antibodies against it.

- This specific immune response also leads to the production of the appropriate memory B and T cells.

- The presence of these memory cells results in long-term immunological memory against the specific antigen

What is herd immunity? (3)

- Herd immunity is a form of indirect protection that occurs when a high proportion of a population is immune to a particular pathogen.

- This provides protection to other people, including those who are not vaccinated, as they are less likely to encounter the pathogen.

- Herd immunity works by reducing the overall spread of a pathogen through a community, which helps to protect the most vulnerable groups in society.

What are three ways pathogens are treated to create a safe vaccine? (3)

- Using a deactivated form of a toxin, which is known as a toxoid.

- Using inactivated (killed) viruses or dead bacteria.

- Using attenuated pathogens, which are alive but have been modified so that they do not cause disease.

Why is vaccination a crucial tool against rapidly acting pathogens? (3)

- The body's natural primary immune response to a new pathogen usually takes several days or weeks to become effective.

- Many pathogens, such as the Ebola virus, can kill an individual in a matter of days, which is too fast for the natural immune response.

- Vaccinations allow the body to be fully prepared to rapidly combat these pathogens, with the peak of the immune response occurring within just two to three days.

How can herd immunity be lost? (2)

- If individuals in a population begin to lose their immunity, for example by not getting vaccinated, it can disrupt the level of protection.

- A loss of herd immunity can lead to a resurgence of diseases that were previously kept under control.