Disease and Pathogens
- A pathogen is an organism that causes disease, including microorganisms (all viruses, some bacteria and fungi) and larger organisms such as tapeworm. They can cause infectious diseases. They can penetrate an organisms surface of contact (interface) with the environment, they need to do this because they need to enter the body to cause disease. There are three main interfaces.
- Gas exchange system - by breathing in air containing pathogens, pathogens can reach the alveoli and invade cells. Most of these are trapped in the mucus lining of the lung epithelium. The cells on the epithelium also have cilia, that brush and move the mucus up the trachea and out of the mouth.
- Skin - damage in the skin can allow pathogens to directly enter your bloodstream. Blood clots at the area of damage usually prevent this, but some get in before the clot has had a chance to form.
- Digestive system - If you eat or drink something that contains pathogen, they are generally killed by the stomachs acidic conditions, but some can survive and travel to the intestines, where they can invade the cells in the gut wall and cause disease.
Pathogens Produce Toxins and Damage Cells
Many bacteria release toxins into the body, which can effect the way the cells function.
They can also damage the host cell by
- Rupturing their membrane, releasing important nutrients such as proteins.
- Breaking down the nutrients in the cell for their own use, this can starve the host cell.
- Replicating inside the host cells (as it contains the necessary components for growth) and bursting out to release the pathogens. Viruses often do this.
Lifestyle can affect the risk of developing diseases e.g Cancer or CHD.
- Cancer is the result of uncontrolled cell division, the risk can be increased by smoking, excessive sunlight exposure and alcohol intake.
- CHD is a disease that affects your heart, the risk can be increased by poor diet, lack of exercise, excessive alcohol intake and smoking.
The Immune System
An immune response is triggered by foreign antigens when they are detected in the body. The immune response occurs in four main stages
(1) Phagocytosis (engulfment of pathogen)
- A phagocyte (type of white blood cell found in blood and on tissues) recognises the antigens on the pathogen.
- The phagocytes cytoplasm engulfs the pathogen, containing the pathogen in a phagocytic vacuole.
- A lysosome (containing lysosomal enzymes) fuses with the phagocytic vacuole and breaks down the pathogen.
- The phagocyte then presents the pathogens antigens by sticking the antigens to it's surface, activating other immune system cells.
T-cells and B-cells
(2) A T-cell is another type of white blood cell, which has proteins on its surface that bind to the antigens which have been presented to it which activates it.
- Some T-cells release substances to activate B-cells
- Others attach to the antigens on the pathogen and kill the cell.
(3) A B-cell is another type of white blood cell, which are covered in antibodies (proteins that bind with antigens to form a antigen-antibody complex) see next card Each B-cell has a different antibody specific to a certain antigen, so they are complementary.
- The antibody on the B-cell surface binds to a complementary shaped antigen.
- This and the substances from the T-cell, activates the B-cell, it then divides into plasma cells.
Plasma Cells and Antibody Structure
(4) Plasma cells are clones of B-cells. They secrete antibodies specific to the antigen.
- Antigens coat the pathogen to make it easier to engulf and to prevent it from entering host cells.
- They also bind to the toxins produced by the pathogen and neutralise them.
The Immune Response
Both the cellular and the humoural response help each other.
- Cellular response- The T-cells and interacting cells such as phagocytes.
- Humoural response- The B-cells and the production of antibodies.
The immune response can be memorised
Vaccines stimulate memory cell production without the person actually getting (or showing any symptoms) a disease. Vaccines contain antigens, which may be free or attatched to dead or weakened (attenuated) pathogens.
- Vaccines contribute towards herd immunity, as vaccines protect the individuals that have them and hence reducing the occurance of the disease. Since the occurance is reduced, people who are not vaccinated are less likely to contract the disease.
- Vaccines are more often injected rather than taken orally. The disadvantage with taking a vaccine orally is that it could be broken down by digestive enzymes or the molecules could be too large to be completely absorbed into the blood. Occasionally, some vaccines need boosters to make sure the memory cells are still being produced.
Antigenic Variation + Flu
Some pathogens change their surface antigens, so the memory cells produced from the original primary response will not recognise the different antigens, and therefore cannot activate the secondary response. This means that the immune system has to carry out another primary response- which takes time to get rid of the infection and the person will get ill again. Antigenic variation makes it difficult to develop vaccines against certain pathogens. Examples of this include HIV and the Influenza virus.
Influenza (Flu)- Proteins on the surface (neuraminidase and haemogglutinin) of the pathogen act as antigens triggering the primary response. These antigens can change regularly forming a new strain each time. This means that memory cells produced from one strain will not recognise all the other strains, forcing the body to carry out another primary response. You can get flu more than once because it's a new strain each time.
Monoclonal antibodies are antibodies produced by a single group of genetically identical cells, so they are all identical in structure. As antibodies are very specific, you can make monoclonal antibodies bind to any desired molecule. These are used in cancer treatment and in pregnancy tests.
Cancer treatment- Cancer cells have antigens called tumor markers that aren't found on normal body cells, so some monoclonal antibodies are made to bind with these. They can also have anti-cancer drugs attached to them, so the drug only accumulates in the body where there are cancer cells. Side effects are lower because the drugs only work in the affected area.
Pregnancy test- They detect the hormone hCG that's found in the urine of pregnant women. The application area contains antibodies for hCG bound to a blue bead, so when urine is applied to the application area any hCG present will bind to the antibody attached to the bead and will form a antigen-antibody complex. The urine then moves up the stick to the test strip carrying any beads with it. The test strip contains immobulised antibodies and if there is any hCG present the beads will turn the test area blue. If no hCG is present then the beads will pass through the area without binding and it won't go blue.
Ethical Issues with Vaccines and Monoclonal Antibo
- Vaccines are tested on animals, some people disagree with animal testing.
- Some contain animal bases substances.
- Testing on humans is compulsory and can be difficult, test volunteers may put themselves at unnecessary risk by thinking they're immune to something when they're not e.g unprotected sex
- Some people don't want a vaccine because of the possible side effects but are protected from herd immunity. Some people think this is unfair.
- If a new epidemic arose difficult decisions would have to be made as to who would be the first and last to be vaccinated.
Monoclonal antibody therapy
- Animals are used to produce the cells from which the monoclonal antibodies are produced, some people disagree.