Pathogens such as viruses, bacteria, protists, and fungi can cause infectious diseases when they enter the body. The immune system provides defenses against pathogens through mechanical barriers like skin and mucus, chemical barriers such as stomach acid, and immune cells that destroy pathogens through phagocytosis and antibody production. Antibodies are proteins that bind to antigens on pathogens. Memory cells provide long-lasting immunity against pathogens the body has previously encountered through vaccination or prior infection. Vaccines work by exposing the immune system to weakened or dead pathogens to stimulate antibody and memory cell production.

1. Pathogens and
immune system
By Victoria Landolfo

2. Pathogens
- Pathogen → microorganism that cause disease
- Many diseases are caused by pathogens that get into our bodies and breed there
EX:
1. Viruses → AIDS
2. Bacteria → cholera
3. Protist → malaria
4. Fungi → athlete's foot
- Diseases (caused by pathogens) can pass from one person to another → called transmissible diseases
- Pathogens damaged our cells by living in them and using up their resources
- Others cause harm to cells by → producing waste products (toxins) which spread around the body and caused
symptoms such as high temperature
- Some toxins are among very dangerous poisons

3. How pathogens enter the body
Direct contact
1. Person to person
2. Blood (like AIDS (virus called HIV)
- Transmission → the passing of a pathogen to an uninfected person
- Infection → the center of a pathogen into the body
- Host → the person in which the pathogen lives and breeds
- Contagious disease → an infectious transmissible disease
Indirect transmission
1. Through air (respiratory passages) → when someone with the illness speaks, coughs or sneezes millions of
viruses are propelled into the air
1. By touching → touch a surface on which they are present and put a hand to your face
2. In food → food can enter you alimentary canal by the food that you ate → if you eat too much you can get food
poisoning
3. Water → if you swim or drink the water that contains these pathogens. EX: bacteria → cholera
By vectors
- A vector is an organism that carries a pathogen from one host to another EX: dog's (rabies), mosquitoes.

4. Body defences
Mechanical barriers
- Structures that make it difficult for pathogens to get past them into the body. EX:
1. Nostrils contain hairs that help to trap dust that might be carrying pathogens
2. Skin has a thick outer layer of dead cells (containing keratin) that is difficult to penetrate → when the skin is cut → blood
clots seal the wound (preventing blood loss and pathogens getting into the body)
Chemical barriers
- Sticky mucus can trap pathogens
1. In the respiratory passages → cilia then sweep the mucus back so it can be swallowed
2. In stomach → hydrochloric acid is secreted → a strong acid that kills many of the bacteria in the food we eat
- If pathogens manage to get through this defences → they are destroyed by white blood cells
- White blood cells can
1. Take in and digest the pathogens by phagocytosis
2. Produce antibodies → vaccination helps antibodies to produce quicklier

5. Antibodies and antigen- Different kinds of lymphocytes produces different kinds of antibodies
- An antibody is a protein molecule with a particular shape
- This shape is just right to fit into another molecule
- To destroy a particular pathogen → antibody molecules must be made which are just the right shape
to fit into molecules on the outside of the pathogen
- These pathogen molecules are called antigens
ANTIGEN: any molecule that is different from the molecules in the cell-surface membranes of our own body
cells and therefore elicits an immune system
- When an antibody molecule lock onto the pathogen → they kill it
- There are several ways in which they do this
1. To alert phagocytes to the presence of the pathogens so that the phagocytes will come and destroy
them
2. Antibodies may start a series of reactions in the blood which produce enzyme to digest the pathogens
- Each lymphocyte waits for a signal that a pathogen which can be destroyed by a particular antibody is
in our body
- If a pathogen enters a body it will meet a lot of lymphocytes
- One of these can recognize the pathogen as being something that its antibody can destroy
- The lymphocyte will start to divide rapidly by mitosis making a clone of lymphocytes
- These lymphocyte then secrete their antibody destroying the pathogen
- The way in which they respond to pathogens by producing antibodies is called the immune system

8. Memory cells
- When a lymphocyte clones itself → not all the cells make antibodies
- Some of the cells simply remain in the blood and other parts of the body → memory cells
- If the same kind of pathogen gets into the body again, these memory cells will be waiting for
them
- They will kill the pathogen before they have time to do any harm
- The person has become immune to that kind of pathogen

9. Vaccinations for immunity
- The vaccine immunise children against diseases caused by pathogens
- A vaccine contains weakened or dead viruses or bacteria that normally causes disease
- These pathogens have the same antigens but they cant cause disease
- These lymphocytes can produce antibodies just as they would after a real infection
- They also make memory cells which give long term immunity

10. Active and passive immunity
- A person has active immunity to disease if they have made their own antibodies and memory cells
that protect against it
- Active immunity → defence against a pathogen by antibody production in the body
You can develop active immunity by:
1. Having the disease and getting over it
2. Being vaccinated by weakened pathogens
- A person has passive immunity if they were given antibodies that have been made by another
organism
- Passive immunity → short term defence against a pathogen by antibodies acquired from another
individual
- Babies get passive immunity by breastfeeding
- Breast Milk contain antibodies from the mother that are passed on to her baby and protects him from
these diseases for the first months of her life
- Another way of getting passive immunity is to be injected with antibodies that have been made by
another organism (ex. antibodies against rabies)
- Active immunity can be very long-lasting while passive immunity lasts for a short time and has no
memory cells