We’ve heard a lot about viruses recently, but how do they work? Viruses cannot live for long without a host. Viruses consist of either a DNA or RNA genome ( genetic code ) and a protein coat, some also have an extra fatty envelope to protect their genetic material. They cannot reproduce without being inside a living cell.
Once inside a cell, viruses use the machinery of the cell to replicate themselves.
Luckily for us the human body has lots of natural responses to protect us from viruses and other harmful pathogens.
How does a virus enter the body?
The human body has several lines of defence against pathogens including viruses. Physical barriers include the skin and mucous membranes. The non specific immune response involves cells from the immune system attempting to destroy pathogens inside the body. Finally the adaptive immune response occurs when a particular pathogen has invaded the body before. Antibodies specific to the invading pathogen are produced to destroy them.
Defence Number 1 – skin and scabs
The first defence against viruses is the skin. Skin is the largest organ of the human body. It helps regulate body temperature, acts as a barrier against bacteria and viruses and is home to 1000s of receptor cells which allow us to feel something when we touch it.
Viruses enter the body through the nose, mouth, eyes or breaks in the skin. When the skin is damaged scabs form over the break to stop pathogens entering.
Make your own clotting blood
This pretend blood recipe actually clots like real blood forming a scab!
Jelly Skin Model
One way to learn about the skin is to make a model of the different layers.
I made this from jelly, liquorice, small sweets and a soft tortilla wrap.
Defence Number 2 – Snot, sweat and saliva
Sticky snot in the nose helps to filter pollutants and pathogens from the air you breathe, preventing them from reaching the lungs. Snot also warms and moistens air before it reaches the lungs.
The next time you think of snot being a bit gross remember how helpful it is!
Did you know sweat, tears and saliva also act as a barrier against pathogens? These all contain an enzyme called lysozyme which can destroy viruses and bacteria.
Make a snot trap
This snot trap is made from cornflour ( cornstarch ) and water. If you sprinkle a bit of cinnamon ( pretend pathogen ) on top, the “snot” will trap it just like the real thing!
Defence number 3 – earwax and nasal hairs
Ear wax also helps trap dust and pathogens before they reach the delicate inner ear!
Defence Number 4 – ciliated epithelial cells
The trachea ( windpipe ) contains lots of mucus and ciliated epithelial cells which force mucus and trapped pathogens back to to the throat to be coughed out of the body or swallowed also preventing them reaching the lungs.
Defence Number 5 – coughs and sneezes
Coughs and sneezes are the body’s way of trying to expel germs from the lungs.
They are also a way pathogens can spread between people. This is why it’s good to cough into your hands or elbow to trap the germs. Just don’t forget to wash your hands afterwards.
Make a sneeze machine
Try adding a couple of tablespoons of water to a balloon using a funnel. Blow up the balloon, hold it up in the air with the end pointing away from you and let the air out. The water will fly out, just like germs escaping from your body when you sneeze!!
Defence number 6 – stomach acid
Pathogens which reach the stomach have a big difficulty to overcome. Acid!!
The stomach contains gastric acid to help break down the food you eat, stomach acid has a pH of around 2 which kills some ( not all ) pathogens.
The stomach has a special lining to stop it becoming damaged by the acid, but if you vomit the acid can make your oesophagus and throat sore.
This activity demonstrating the path of food through the human digestive system uses orange juice as the stomach acid.
Defence 7 – Inside the body
If a pathogen manages to overcome the defences on the outside of the body the cells of the immune system kick in to destroy them.
White Blood Cells
White blood cells ( leukocytes ) are constrantly travelling around the body in the blood searching for new pathogens to destroy. There are several different types of white blood cells.
Make a blood cocktail
This blood cocktail is a fun and tasty way to demonstrate the what’s in blood.
I used soda water for the plasma, pomegrante seeds for red blood cells, banana for white blood cells, and pineapple for platelets.
For every 1 white blood cell there are about 40 platelets and 600 red blood cells which is why the drink is mostly pomegranete!
Lymphcytes produce antibodies. Antibodies act in several different ways.
- Bind to pathogens allowing them to be recognised by phagocytes which can destroy them.
- Clump pathogens together so they can be more easily destroyed.
- Neutralise toxins.
There are two types of lymphocytes both are made in the bone marrow.
B lymphocytes find foreign cells and attach to them, while T lymphocytes destroy the invaders found by the B cells.
B cells are the ones that produce antibodies.
Phagocytes engulf and destroy pathogens by surrounding them in a vacuole and then secreting enzymes to destroy them. This process is called phagocytosis.
What’s the difference between phagocytes and lymphocytes?
Lymphocytes mount a specific targeted immune response to pathogens ( adaptive immunity ).
Phagocytes have a more general response and engulf and destroy all pathogens in the same way ( innate immunity ).
If you want to read more about immunology ( we’ve only scratched the surface here ) there are some great resources on the British Society for Immunology website.
What does it mean if you are immune to a virus?
We say someone is immune to a virus or bacteria when they have antibodies in their blood ready to fight the pathogen as soon as it enters the body. As soon as the immune system recognises the pathogen it creates antibodies to fight it very quickly, so the infected person probably doesn’t even realise they were infected!
Immunity can be natural ( after an initial infection ) or acquired ( from a vaccine ).