Infection and Response

Diseases

pathogens are microorganisms that cause infectious diseases

bacteria causes disease by producing toxins that affect the body and reproducing via binary fission to cause this damage on a large scale. examples of bacterial diseases are:

  • salmonella: picked up when food is prepared incorrectly, causing fever, vomiting, stomach pain and cramps. cook food thoroughly + in the UK, poultry is vaccinated against it

  • gonorrhoea: an STD. it causes pain when urinating and a yellow/green discharge. control with use of antibiotics if infected and barrier methods

viruses inject a cell with their DNA and cause them to lyse. when they do, they spread that DNA to other cells and cause them to lyse and die.

  • measles: spread by inhaling droplets that contain it. it causes fever and rash, but can be fatal if complications arise. it’s generally vaccinated against at an early age

  • HIV: spread via bodily fluids like semen and blood. it causes flu-like symptoms but then attacks the immune system, developing into AIDS. prevent be using barrier methods and not sharing needles

  • TMV: spreads via direct contact. causes a mosaic-like pattern of discoloration on leaves which stunts growth, because it kills cells with chlorophyll used in photosynthesis that gives leaves their green colour. there’s no cure, so you’ll just have to remove the infected part

fungi are organisms that are made of structures that can penetrate tissues

  • rose black spot: spreads in water or wind. causes purple/black spots on rose plant leaves, eventually making them fall off and thus preventing growth because rate of photosynthesis drops. use fungicides and destroy affected leaves

protists are single celled eukaryotic organisms, and are often parasites, meaning that they rely on other organisms to be transferred into a host and cause damage.

  • malaria: spread by a certain type of female mosquito. when an infected mosquito lands on the skin, it transfers the infection to the blood. it causes recurring fevers, nausea, anaemia and jaundice, and can be fatal. use mosquito nets and prevent breeding of carrying mosquitos.

Human defence systems

non-specific defence systems:

  • skin: acts as a physical barrier (unless there’s a break) and houses skin flora that competes with bad microorganisms for space and nutrients, hopefully winning

  • platelets in blood: cause blood to clot and eventually scab around breaks in skin, preventing pathogens from entering the bloodstream

  • nose: has hairs and mucus that stop infected particles from entering the lungs

  • trachea and bronchi: secrete mucus to trap pathogens and have hair-like cilia that sweep bad substances out

  • stomach: hydrochloric acid kills microbes in food and drink

white blood cells:

there’s two types of white blood cells that combat pathogens.

1. general white blood cells, or phagocytes, target any foreign bodies via phagocytosis. this is where they surround, engulf and digest the foreign body so they no longer cause illness

2. specific white blood cells, or lymphocytes, target one specific pathogen each. they can further be subdivided into:

  1. antibody lymphocytes that are specific and complementary to an antigen (basically an identification feature) on the surface of a pathogen. when they bind together to form an antibody-antigen complex, pathogen cells get clumped together, making phagocytosis easier and thus causing less damage to cells.

    • while this process shares a lot of similarities with enzymes, it’s NOT A CHEMICAL REACTION.

  2. antitoxin lymphocytes neutralise toxins released by a pathogen when they bind to them.

Vaccinations

vaccinations involve injecting a small amount of a dead or inactive form of a pathogen into the bloodstream to trigger an immune response and stimulate the white blood cells to produce the right antibodies to combat it.

this is usually done for diseases where they can kill you before your white blood cells can prepare the specific and complementary antibodies. so by vaccinating, if the pathogen re-enters the body the white blood cells can respond quickly, already having a response, and thus preventing illness

herd immunity: when a large proportion of a population is immune to a pathogen, reducing its spread and even potentially making it disappear

Antibiotics and Painkillers

painkillers relieve symptoms but don’t kill pathogens

antibiotics cure bacterial diseases, but don’t affect viruses. bacteria can, over time, become resistant to antibiotics. to prevent this, there’s a few things you can do:

  • only give them to people in exceptional circumstances

  • ensure people finish their course so that the bacteria doesn’t build up again and they have to come back for more

  • use a variety of antibiotics to treat a bacterial disease

links to culturing microorganisms, discussed in cell biology notes. we culture microorganisms at 25 degrees C in a school, but something closer to 40 degrees may be optimal in a lab setting so they reproduce faster and we can get an accurate view of how they’d respond to a temperature around human body temp

Drug discovery, Development and Trialling

drugs were, initially, mostly discovered and extracted from plants. for example aspirin originates from willow plants. nowadays, most new drugs are synthesised in labs, but plants may be used as a starting point for the types of molecules we’re looking to build.

preclinical testing should be carried out on cells, tissues and animals to test for toxicity, efficacy and dose before a human ever receives the drug.

clinical trials have a few steps and variations.

phase 1: low doses of the drug are given to healthy people to make sure it’s not toxic

phase 2: a small group of people with the disease take the drug to test its efficacy vs a placebo. this could be done in a few different ways:

  • open trial: both doctor and patient know if they’re taking the drug

  • single blind trial: doctor knows what the patient has but the patient doesn’t know what they have

  • double blind trial: neither doctor nor patient know what they have.

if people feel well after taking the placebo, then the drug mustn’t be very effective. in single blind and open trials, there’s potential for bias from a doctor to focus on certain groups in a report, so double blind trials are most commonly used.

phase 3: a large amount of people with the disease are given the drug to nail down its dosage, widen the sample size to better understand anomalies and determine any side effects.

Monoclonal Antibodies

production

  1. an animal like a mouse is injected with an antibody

  2. it’s B-lymphocyte wbcs are stimulated to produce antigens

  3. B-lymphocytes are isolated from spleen

  4. cancerous myeloma cells are added to the lymphocytes to produce a hybridoma cell, which can divide quickly and produce antibodies

  5. the best hybridoma cells are selected and cultured

  6. their antibodies are isolated and purified for use in humans

uses

to deliver drugs to a cancer cell that will destroy it

to block receptors than enable cancerous cells to multiply

bound to pathogens and then to a fluorescent dye so pathogens can be observed in a lab

pregnancy tests - this works on the idea that a pregnant woman’s urine will contain the hormone hCG, which monoclonal antibodies can bind to.

  1. a woman urinates on the stick. if she’s pregnant, the hCG in her urine will bind to a monoclonal antibody with a dye attached to it, and move up the stick

  2. in the test window, there are immobile antibodies that bind to hCG (and thus by extension the dye). if you’re pregnant, the hCG-antibody complex will bind it the immobile antibodies and the dye will show. if not, no line is shown

  3. most pregnancy sticks have two lines. the second is an immobile antibody complementary to the monoclonal antibody, and is used as a control to make fairly sure the test is working correctly.

pros

cons

only bind to specific cells, so healthy cells not affected

expensive

can be adapted to treat many different conditions and have many use cases

triggered more side effects in humans that initially thought

Disease in plants

Plant diseases can be detected by:

  • stunted growth

  • spots on leaves

  • areas of decay (rot)

  • growths

  • malformed stems or leaves

  • discolouration

  • the presence of pests.

Identification can be made by:

  • reference to a gardening manual or website

  • taking infected plants to a laboratory to identify the pathogen

  • using testing kits that contain monoclonal antibodies

ion deficiencies cause problems in plants. there’s 2 you need to know:

1. nitrate ion deficiency: nitrogen binds with glucose to make amino acids that form into proteins needed for growth. it’s naturally found in soil. it can’t just be taken from the atmosphere due to strong covalent bonds between these nitrogen atoms

2. magnesium ion deficiency: magnesium is required to make chlorophyll, so when there’s not enough of it leaves turn yellow and the rate of photosynthesis in the leaf decreases

plants have a number of defences against disease.

physical defences:

  • cellulose cell walls are a physical barrier into the cell

  • waxy cuticle stops entry into the leaf

  • dead cells around stem/bark on trees stops pathogens from entering by falling off and taking pathogens with them

chemical defences:

  • poisons that deter herbivores

  • anti-microbial properties that kill microbes (and are also extracted by humans for our use)

mechanical defences:

  • thorns that make it painful for an animal to eat the plant

  • hairs that make it impossible for insects to lay eggs. when these eggs hatch into larvae, they eat into the leaf, leaving a break in it and making it vulnerable to pathogens

  • leaves droop/curl when touched to scare bugs that land on them off

  • mimicry - plants make themselves look like unhealthy plants, other objects like rocks or even animals to deter animals from eating them.