IB Biology HL - Lesson 3: C3.2 Defence Against Disease

pathogens

organisms that cause disease, are the 'starting points' in the chain of infection.

what do pathogens include?

types of bacteria, fungi, protists, viruses and prions.

bacteria as pathogens

• as they are prokaryotes, they have a different cell structures different from the eukaryotic organisms they infect. they are classified by their shape and their cell walls.

• the type of membrane affects how bacteria responds to antibiotics

viruses

• non-living infectious agents, have proteins surrounding RNA.

• they infect cells by inserting its RNA into the DNA of a cell, and the infected cell will begin to form more viruses.

• all natural viruses are pathogens, and may affect all types of organisms, even bacteria (bacteriophages)

protists as pathogens

• eukaryotic organisms that are either single celled or are grouped into small colonies.

• only a small percentage of protists are pathogenic, but can cause serious disease (malaria, fx)

• pathogenic protists often need a vector to transfer them to their host - they take over a cell, digest the insides and use it as a medium to reproduce

fungi

• mainly effect plants. they are eukaryotes, but can be multi/unicellular.

• many fungi feed of dead/decaying matter (saprophytes), and typically prevent photosynthesis as they infect plant leaves.

• they produce millions of spores that rapidly infect other organisms

prions

• infectious proteinaceous substances that are responsible for diseases that cause degeneration of the nervous system in mammals.

• (mad cow disease/BSE)

how do pathogens effect organisms?

• often damage tissues or produce toxins.

• in conjunction with the immune response creates symptoms of an infection

how do bacteria damage tissues?

will produce a toxin which damages the cell of the host organism - by either breaking down the plasma membrane of inactivating enzymes.

non-specific defences

1. surface barriers

2. innate immunity

specific defences

adaptive immunity

surface barriers as a non-specific defence

e.g. skin, skin flora, mucous membranes and lysozymes (enzymes in tears, urine and stomach acid)

the skin as a non-specific defence

• covers the entire body and seals the insides.

• sebaceous glands on the skin produce oils which keep the skin at a slightly lower pH, preventing growth of bacteria - producing a chemical barrier.

mucous membranes as non-specific defences

• they line the body cavities and areas which have access to the outside, like the digestive, urogenital and respiratory tracts, salivary ducts

• the mucus produced traps pathogens, and contain an enzyme called lysozyme which attacks the bacterial cell walls.

• mechanical actions, like sneezing also flushes away the mucus with trapped pathogens using things like cilia.

commensal bacteria (skin flora) as a non-specific defence

• microbes that reside on the surface of the body or in mucous that dont harm human health.

• the microbes live in harmony with humans as they prevent the production of other bacteria as they digest it.

blood clotting process

1. when a cut is formed clotting factors released

2. platelets clot at the site of the cut

3. soluble fibrinogen is made into insoluble fibrin by the prothrombin which is activated into thrombin using the clotting factors.

the fibrin produced from the fibrinogen (using thrombin) clots to form a mesh.

4. the clot develops into a scab, and more layers of skin are formed underneath.

innate immune systems

present from birth and provides rapid, non-specific defences against pathogens and is mediated using phagocytes.

adaptive immune systems

develops throughout life and is slower and specific. mediated by lymphocytes (antibodies)

phagocytes

a type of white blood cell that ingests invading microbes

they come in two main types: neutrophils and macrophages

phagocytosis

1. pathogens produce chemicals which attract phagocytes

2. phagocytes recognise non-human proteins on the pathogen and form a pseudopodia

3. the phagocyte engulfs the pathogen into a phagosome

4. the phagosome combines with a lysosome to form a phagolysosome

5. enzymes in the lysosome digest and destroy the pathogen.

neutrophils end their cycle here

phagosome

intracellular vesicle containing material taken up by phagocytosis.

lymphocytes

The two types of white blood cells that are part of the body's immune system:

B lymphocytes -- mature on the bone marrow

T lymphocytes -- mature in the thymus gland

B lymphocytes

responsible for producing antibodies. they also form plasma cell clones.

mature in the bone marrow and are part of the immune response.

T lymphocytes

they assist in activating B cells while cytotoxic T cells directly attack infected cells and cause cell lysis.

mature in the bone marrow and are part of the immune response.

antigens

• molecules on top of cells which are different between different cells (glycoproteins).

• the body can differentiate between self-antigens and non-self antigens on pathogens.

• antigens trigger an immune response, which involves the production of antibodies

antibodies

• y-shaped proteins called immunoglobulins and bind to a specific antigen that triggers the immune response. they are made up from two identical polypeptide chains which are attached to each other and two short identical light chains by disulphide bridges.

• proteins that bind to antigens on pathogens which marks them for destruction through the lock and key mechanism (B cells)

how do antibodies work in an immune response?

• a pathogen enters the body with specific antigens.

• these antigens are recognised by B cells and the B cells differentiate to plasma cells, producing antibodies. these antibodies are specific to an antigen.

• the antibodies bind to the pathogen and go through the process of neutralisation where the pathogen's infectious ability is blocked.

• antibodies also can activate the complement system, which leads to the destruction of the pathogen and also enhance the activity of phagocytes

• aglutination

glutination

clotting, clumping or coagulation. antibodies agglutinate when they meet an antigen

T killer cells

T lymphocytes that destroy pathogens carrying a specific antigen with perforin.

T memory cells

part of immunological memory

divide rapidly to produce large number of T killer clones when met with the antigen a second time

T regulator cells

regulate the immune system. they stop the immune response once a pathogen has been eliminated and makes sure the body recognises self antigens

T helper cells

have CD4 receptors that bind to the antigens on the antigen presenting cells, producing interleukins (cell signaling molecules)

interleukins

stimulate the activity of B cells (increasing antibody production)

stimulate the production of other T cells

attracts macrophages to ingest pathogens

macrophages

found within the lymph nodes, they are phagocytes that destroy bacteria, cancer cells, and other foreign matter in the lymphatic stream to form APC

plasma cells

produce antibodies and release them into circulation

B effector cells

these are the B lymphocytic cells that can divide to form plasma cell clones

B memory cells

live for a very long time and provide immunological memory

programmed to remember specific antigen and enable rapid response when antigen encountered again

activation of T-lymphocytes by helper T cells

1. helper T-cells bind to the antigen presented on the antigen-presenting cells

2. the helper T-cell recognises the antigens and becomes activated (secretes cytokines) to signal other immune cells

3. activated T-cells also help activate cytotoxic (killer T-cells) by providing cytokine signals

4. helper T-cells require additional signals from co-stimulatory molecules to active the immune response

APC

antigen presenting cell -- has antigens from engulfed pathogens which helper T-cells will identify and prompt an immune response

humoral immunity

specific immunity produced by B cells that produce antibodies that circulate in body fluids

memory cells

differentiate into plasma cells which produce antibodies when an old pathogen is re-introduced which triggers a much quicker immune response

why are people with the AB blood type considered to be universal recipients?

it has both A and B antigens, but no antibodies as it has all the antigens. so no agglutination occurs when all blood types are introduced.

why are type O universal donors?

it has no antigens on the red blood cell, but all the antibodies.

ways HIV is transmitted

bodily fluids, breast milk and urine

features of HIV

envelope around the capsid

two identical strands of RNA within the capsid, contains reverse transcriptase which allows the production of DNA from the RNA code and is referred as a retrovirus as it can make DNA from RNA. Spikes on the surface are made from proteins and carbohydrates

effect of HIV on the immune system

it attacks the helper T-cells, making them reproduce far less - decreasing immunity responses and making the people infected have a higher probability of being immunocompromised

stages of untreated HIV

HIV infects helper T-cells and they continue reproducing until the virus count is higher than the number of T-cells (CD4 cells) and may last several years. when the HIV-infected cells are at a higher count, they have AIDS and are at a risk for diseases

antibiotics

a group of drugs used to treat bacterial infections. they are effective in killing prokaryotic cells (e.g. bacterial infections) but generally leave eukaryotic cells unharmed.

some antibiotics prevent the formation of bacterial peptidoglycan cell walls -> osmotic lysis

bactericidal antibiotics

antibiotics that kill bacteria

bacteriostatic

slow the growth or reproduction of bacteria

antibiotic resistance

in a population with no antibiotic resistance, there is a chance that a bacterium produces a gene for antibiotic resistance.

when an antibiotic is added, the resistant bacteria may still proliferate as they have greater resources to do so, as the non-resistant strain had been eradicated.

therefore, a population with mostly resistant bacteria is formed.

zoonotic diseases

transmission of disease via animals to humans (e.g. rabies, brucellosis, COVID-19)

how do vaccine develop immunity?

1. the pathogen is made safe (e.g. a dead/infected pathogen, RNA/DNA from pathogen)

2. and the pathogen is injected into the body

3. an immune response is provided and the body produces memory cells

herd immunity

when a large proportion of the population becomes resistant to a disease through vaccination, creating a barrier to those who could not get the vaccine