Microorganisms & Immunity (Edexcel A Level Biology (A) SNAB): Flashcards

HIV is the virus.

AIDS is the disease it eventually causes when helper T cell numbers drop too low.

An opportunistic infection is one that takes hold because the immune system is weakened, such as TB in an AIDS patient.

HIV uses the enzyme reverse transcriptase to convert its RNA into DNA.

False.

All viruses are pathogens in the sense of needing a host, but not all viruses cause disease in humans.

Broken skin.

The digestive system and the respiratory system.

Mucosal surfaces.

It is a physical barrier covering the body.

If damaged, the blood clotting mechanism helps seal the wound.

Hydrochloric acid creates an acidic environment.

This is unfavourable to many pathogens on food and drink.

Lysozyme is an enzyme in mucosal secretions (tears, saliva, mucus) that damages bacterial cell walls, causing them to burst.

They compete with pathogens for resources.

This limits pathogen numbers and their ability to infect.

In secretions of the mucosal surfaces.

It breaks down bacterial cell walls, causing the bacteria to lyse.

It provides direct access to the tissues and bloodstream, bypassing the physical barrier.

The lining of the nose, mouth or genitals.

The enzyme lysozyme, found in tears and saliva, damages bacterial cell walls.

False.

Clotting takes time, so a few pathogens may enter before a clot forms.

A few may survive the acid.

They reach the intestines and infect the gut wall cells.

Gut flora are the harmless microorganisms living in the gut that compete with pathogens and help limit infection.

A non-specific response is the same for any pathogen.

A specific response is targeted at a particular pathogen.

Inflammation, interferons and phagocytosis.

Mast cells release histamine.

It causes vasodilation and makes capillaries more permeable, producing swelling.

Inflammation is the swelling, warmth and pain around a wound, caused by histamine increasing blood flow and capillary permeability.

Interferons are antiviral proteins made by virus-infected cells that stop viruses spreading to other cells.

They inhibit viral protein production, stopping the virus replicating.

They activate white blood cells of the specific immune response.

Phagocytosis is the engulfing and digesting of pathogens and dead cells by phagocytes.

A phagocyte is attracted to and recognises the pathogen's antigens as non-self.

It engulfs the pathogen into a vacuole, where lysosomes release enzymes to digest it.

It presents the pathogen's antigens on its cell surface membrane.

It becomes an antigen-presenting cell, initiating the specific immune response.

They digest the pathogen trapped in the phagocytic vacuole.

One example is lysozyme.

During inflammation, mast cells release histamine, which makes capillaries more permeable.

False.

The non-specific response begins immediately when a pathogen invades.

An antigen is a molecule (often a protein or glycoprotein) on a cell surface that acts as an ID marker for cell recognition.

Self antigens are made by the organism's own cells and do not trigger an immune response.

Non-self antigens (e.g. on pathogens) do trigger an immune response.

An antigen-presenting cell is a phagocyte that displays a digested pathogen's antigens on its surface, activating the specific immune response.

A Y-shaped molecule of four polypeptide chains — two heavy and two light — held by disulfide bonds.

Each chain has a constant and a variable region.

It contains the antigen binding site.

Its amino acid sequence varies, giving each antibody its specificity for one antigen.

An antigen-antibody complex is the structure formed when an antibody's variable region binds to its specific antigen.

Immunoglobulins.

Agglutination is the clumping together of pathogens by antibodies, which stops them spreading and helps phagocytes engulf many at once.

They block receptors, preventing pathogens infecting host cells.

They act as anti-toxins, and cause agglutination.

Clumped pathogens cannot spread easily through the body.

Phagocytes can then engulf many pathogens at once.

The part of an antibody that binds to an antigen is the variable region.

False.

Self antigens do not trigger a response; only non-self antigens do.

A T cell (T lymphocyte) is a white blood cell of the specific immune response that matures in the thymus.

They are produced in the bone marrow.

They mature in the thymus (the source of the 'T').

Its T cell receptor binds to its specific antigen.

The antigen is displayed on an antigen-presenting cell.

To produce many genetically identical clones.

All carry the same T cell receptor, specific to that antigen.

T helper cells, T killer cells and T memory cells.

It releases chemical signalling molecules (cytokines).

These help to activate B cells.

It binds to and destroys infected cells displaying the specific antigen.

It remains in the blood.

It enables a faster specific immune response if the same pathogen returns.

An antibody.

Each receptor is specific to a particular antigen.

T cells mature in the thymus, which is where the 'T' in their name comes from.

False.

T killer cells destroy infected cells; antibodies are produced by plasma cells from B cells.

A macrophage, an infected body cell, or the pathogen itself.

A B cell (B lymphocyte) is a white blood cell of the specific immune response that matures in the bone marrow.

Antibodies, known as antibody receptors.

Each B cell has a different type, specific to one antigen.

The B cell binding to its specific antigen.

Signalling molecules from T helper cells.

It divides repeatedly by mitosis.

The clones differentiate into plasma cells and memory cells.

They produce specific antibodies to combat the non-self antigen.

They remain in the blood.

They allow a faster immune response if the same pathogen returns.

An antigen-antibody complex.

So that between them, B cells can recognise a huge range of different antigens.

The right B cell can then respond to any pathogen.

Activated B cells differentiate into memory cells and plasma cells, which produce antibodies.

False.

B cells mature in the bone marrow (the source of the 'B'); T cells mature in the thymus.

Mitosis, producing many identical clones.

To produce plasma cells that make antibodies against the specific antigen.

And memory cells for long-term immunity.

An intron is a non-coding section of DNA within a gene, removed from pre-mRNA during splicing.

An exon is a coding section of DNA within a gene, which is kept and joined together during splicing.

Splicing is the removal of introns from pre-mRNA and the joining of exons, producing mature mRNA before it leaves the nucleus.

It happens after transcription.

The pre-mRNA is modified before it leaves the nucleus.

Only the coding sequences (exons) of the gene, joined together.

Alternative splicing is the joining of exons in different combinations, so one gene can code for more than one polypeptide.

The antibody heavy-chain gene can be spliced in different ways.

This produces either a membrane-bound or a secreted antibody.

An extra section of polypeptide.

This allows the antibody to attach to the surface of a white blood cell.

By alternative splicing — its exons are joined in different combinations to make different mature mRNAs.

During splicing, non-coding introns are removed and coding exons are joined together.

False.

Exons are the coding sections; introns are the non-coding sections.

Pre-mRNA, which contains both introns and exons.

There are few T and B cells with the correct receptor at first.

They need time to be activated, divide and differentiate into plasma cells.

Memory cells from the primary response are already present in large numbers.

They detect the antigen and produce antibodies quickly.

Active immunity is immunity gained when an antigen triggers your own immune response, producing memory cells.

Passive immunity is immunity gained by receiving antibodies from another source, without your own immune response.

Active immunity produces memory cells and is long-lasting.

Passive immunity has no memory cells and is short-lived.

Antibodies passed from mother to baby across the placenta or in breast milk.

Natural is from being infected by a pathogen.

Artificial is from a vaccination.

It contains antigens (e.g. weakened or dead pathogens).

It gives artificial active immunity.

The vaccine made memory cells.

These trigger a fast, strong secondary response that clears the pathogen before symptoms appear.

Immunity gained by receiving ready-made antibodies from another source is called passive immunity.

False.

Passive immunity is short-lived because no memory cells are produced.

Pathogens undergo antigenic variation through mutation.

The vaccine must be updated to match the changed antigens.

The evolutionary race is the ongoing battle in which hosts evolve better immune defences and pathogens evolve ways to evade them.

It reduces the number of cells that can detect the virus and activate antibody production.

The immune response is weakened.

Antigenic variability is the frequent change of a pathogen's surface antigens due to mutation, forming new strains.

Each new strain has different antigens.

Memory cells for one strain do not recognise another, so a new primary response is needed each time.

It prevents infected cells from presenting antigens on their surface.

This makes it hard for white blood cells to recognise and destroy them.

It produces substances that stop lysosomes fusing with the phagocytic vacuole.

So it is not broken down and can multiply inside the phagocyte.

They both disrupt antigen presentation in infected cells.

This makes the infected cells hard for the immune system to detect.

Each side keeps evolving new advantages over the other.

Hosts improve defences and pathogens improve evasion, continually.

HIV's high mutation rate causes antigenic variability, forming new strains the immune system does not recognise.

False.

Memory cells for one strain do not recognise the different antigens of another strain.

They block the lysosome from fusing with the vacuole.

Without digestive enzymes, they survive and reproduce.

They show pathogens have evolved specific ways to overcome host defences.

This is the pathogen's 'move' in the ongoing arms race.

An antibiotic is a chemical that damages bacterial cells with little or no harm to human tissue.

Bactericidal antibiotics kill bacteria.

Bacteriostatic antibiotics inhibit bacterial growth.

It inhibits enzymes needed to build the cell wall.

The weakened wall bursts as water enters by osmosis, lysing the cell.

Binding to ribosomes to stop protein synthesis.

Damaging the cell membrane or preventing DNA coiling.

Human cells are eukaryotic — they have no cell wall, and different enzymes and ribosomes from bacteria.

Viruses lack the structures antibiotics target — cell walls, ribosomes and enzymes.

Aseptic technique is working with sterile equipment in a sterile environment to prevent contamination by microorganisms.

An area where bacteria could not grow.

A larger clear zone means a more effective antibiotic.

Distilled water.

It shows any effect is due to the antibiotic, not the paper disc or another factor.

It creates an updraft.

This stops airborne microorganisms landing on the plate and contaminating it.

25 °C avoids growing harmful human pathogens.

Light taping lets oxygen in for the bacteria while limiting contamination.

An antibiotic that kills bacteria is bactericidal, whereas one that inhibits their growth is bacteriostatic.

True.

If the bacteria grow right up to the disc, they are resistant to that antibiotic.

A hospital acquired infection is an infection a patient contracts while in hospital.

Methicillin-resistant *Staphylococcus aureus*.

It is resistant to the antibiotic methicillin, so does not respond to regular antibiotics.

Antibiotics act as a selection pressure.

Resistant bacteria survive and reproduce, so resistance increases by natural selection.

They are already ill and often have weakened immune systems.

This makes them more likely to catch infections.

Staff and visitors washing hands regularly.

Disinfecting surfaces and equipment, and isolating infected patients.

Unnecessary use adds a selection pressure for resistant bacteria.

Antibiotics also do not work against viruses.

It targets a narrow range of bacteria.

Any resistance genes that arise are less likely to cause problems in other bacteria treated with different antibiotics.

It reduces the chance of bacteria developing resistance to any one antibiotic.

By horizontal gene transfer.

This lets resistance spread between bacteria, even of different types.

An antibiotic effective against only a small range of bacteria is called a narrow-spectrum antibiotic.

True.

Resistant bacteria survive antibiotic treatment and pass on the resistance, so it spreads by natural selection.

It exposes bacteria to antibiotics unnecessarily.

This increases the selection pressure for resistant strains to develop.