Disease and Immunity

Pathogens cause infectious disease.

A pathogen is any organism that causes a disease. They include microorganisms as well as larger organims like tape worm.

Pathogenic microorganisms include some bacteria, some fungi and all viruses.

Pathogens have to enter the body in order to cause disease. 

Gas exchange system - when you breathe in air that contains pathogens, most of them will become trapped in the mucus lining the lung epithelium. These cells also have cilia that beat and move the mucus up the trachea to the mouth where it is removed. Some pathogens are able to reach the alveoli where they can invade and damage the cells.

Skin - if the skin is damaged, pathogens on the surface can enter into the bloodstream. The blood clots at the damaged area to prevent pathogens entering but some may enter before the clot takes place.

Digestive system - if you eat or drink food that contains pathogens most of them are killed by the acidic contions of the stomach. However, some will survive where they move into the intestine and begin to invde cells. 

Pathogens cause diease by producing toxins and damaging cells.

Production of toxins: Many bacteria release toxins into the body.

Cell damage: Pathogens can physically damage host cells.

Rupturing them to release nutrients inside them.

Breaking down nutrients inside the cell for their own use. This starves and eventually killed the cell.

Replicating inside of cells and bursting them when they're released.

CHD - poor diet, smoking, lack of eercise, high blood pressure.

Cancer (uncontrolled cell division) - smoking, excessive exposure to sunlight, excessive alcohol consumption.

Foreign antigens trigger an immune response.

Phagocytes engulf pathogens. Theyu are a type of white blood cell that carry out phagocytosis.

A phagocyte recognises the antigens on a pathogen.

The cytoplasm of the phagocyte moves around the pathogen engulfing it.

The pathogen is now contained in a phagocytic vacuole in the cytoplasm of the phagocyte.

A lysosome fuses with the phagocytic vacuole. The lysosomal enzymes break down the pathogen.

The phagocyte then presents the pathogen's antigens - it sticks the antigens on the surface to activate other immune systems.

Phagocytes activates T-cells.

A T-cell is another type of white blood cell. It has proteins on its surface that bind to the antigens presented to it by phagocytes.

This activates the T-cells.

Some release substances that activate B-cells.

Some attach to antigens on a pathogen and kill the cell.

T-cells activate B-cells which then divide into plasma cells. The B-cells are covered in antigens to form an antigen-antibody complex. Each B-cell has a different shaped antibody on it's membrane so different ones bind to different shaped antigens.

When the antibody on the surface of a B-cell meets the complementary shaped antigen, it binds to it.

This together with the substances from the T-cells, activates the B-cell. The B-cell then divides into plasma cells.

Plasma cells make more antibodies to a specific antigen.

Plasma cells are indentical to the B-cell. They secrete loads of antibody specific to the antigen. Antiboy functions include:

Coating the pathogen to make it easier for a phagocyte to engulf it.

Coating the pathogen to prevent it from entering host cells.

Binding to the neutralising toxins produced by the pathogen.

Immume response is split into cellular and humoral.

Cellular - the T-cells and other immume system cells that they interact with.

Humoral - B-cells and the production of antibodies from the humoral response.

Primary Response

When an antigen enters the body for the first time it activates the immune system. This is called the primary response.

The primary response is slow because there aren't many B-cells that can make the antibody needed to bind to it.

Eventually, the body will produce enough of the right antibody to overcome the infection. Meanwhile the infected person will show symptoms of the disease.

After being exposed to the antigen, both T and B-cells produce memory cells. These memory cells remain in the body for a long time. Memory T-cells remember the specific antigen and will recognise it a second time round. Memory B-cells record the specific antibodies needed to bind to the antigen.

The person is now immune.

The secondary response

If the same pathogen enters the body again, the immune system will produce a quicker, stronger immune response.

Memory B-cells divide into plasma cells that produce the right antibody to the antigen.

Memory T-cells divide into the correct type of T-cells to kill the cell carrying the antigen.

The secondary response often gets rid of the pathogen before you begin to show any symptoms.

Vaccines can protect induviduas and popluations against disease.

WHile your B-cells are busy dividing to build up their numbers to deal with a pathogen, you suffer from the disease. Vaccination helps to avoid this.

Vaccines contain antigens that cause your body to produce memory cells against a particular pathgogen, without the pathogen causing disease. This means you become immune without getting any symptoms.

Vaccines protect induviuals that have them and, because they reduce the occurence of the disease, those people that are still not vaccinated are less likely to catch the disease. This is called herd immunity.

Vaccines always contain antigens - these may be free or attatched to a dead or inactive pathogen.

Vaccines may be injected or taken orally. Disadvantages of taking it orally may be that it is broken down by enzymes in the gut or the molecules may be too large to absorb into the blood stream.

Sometimes booster vaccines are given later on to make sure that memory cells are produced.

Antigenic Variation helps some pathogens evade the immune system.

Antigens of the surface of pathogens activate the primary response.

When you are infected a second time with the same pathogen they activate the secondary response and you don't get ill.

However, some pathogens change the antigens on the surface. This is called antigenic variation.

This means that when you are infected for a second time, the memory cells produced from the first infection will not recognise the different antigens. As a result, the immune system has to start from scratch and carry out primary response to these new antigens.

This takes longer to get rid of the infection which is why you become ill again.

Antigenic variation also makes it difficult to develop vaccines against some pathogens for the same reason.

Antigenic variation in the influenza virus

Proteiens on the surface of the influenza virus acts as antigens triggering the immune system.

These antigens can change regularly, forming new strains of the virus.

Memory cells produced from infection with one strain of flu will not recognise other strains with different antigens.

This means your immune system produces a primary response every time you are infected with a new strain.

This consequently means you can suffer from flu multiple times.

Monoclonal antibodies are antibodies produced from a single group of genetically identical B-cells. This means that they are all identical in structure.

Antibodies are very specific because their binding sites have a unique structure that only one antigen will fit.

You can make monoclonal antibodies that bind ot anything you want and they will only target these molecules.

Cancer

Cancer cells have antigens called tumour markers that are not found on normal body cells. Monoclonal antibodies can be made that will bind to the tumour markers. You can also attatch anti-Cancer drugs to these antibodies.

When the antibodies make contact with the Cancer cells they bind to the tumour markers.

This means the drug will only accumulate where there are cancer cells. This means side effects are lower than other drugs as they are cell specific.

Pregnancy Tests

Pregnancy tests detect the hormone hCG that's found in the urine of preganant women.

The application area contains antibodies for hCG bound to a blue bead.

When urine is applied to the appliction area any hCG will bind to the antibody on the beads forming an antibody-antigen complex.

The urine moves up to the test ***** carrying any beads with it. The test ***** contains antibodies to hCG that are immobilised.

If there is any hCG present the test ***** turns blue because the immobilised antibody binds to any hCG. If no hCG is present the beads will pass through the test area without binding to anything and therefore not turning the ***** blue.

Vaccines

All vaccines are tested on animals before being tested on humans.

Animal based substances may be used to create substances.

Testing vaccines on humans is risky.

Some people may not want the vaccine due to the risk of the side effects but are still protected due to herd immunity.

Decisions about who recieves the vaccine. Eg. a new epidemic would create a rush.

Monoclonal Antibodies

Animals are used to produce monoclonal antibodies.