Disease & Immunity

• A pathogen is any organism that causes disease.
• Pathogens cause disease by producing toxins and damaging cells.
• Pathogens can physically damage host cells by:
  • Rupturing them to release nutrients.
  • Breaking down nutrients, which starves and kills the cell.
  • Replicating inside the cell, causing them to burst.

Pathogens can enter the body in 3 ways:

• Respiratory System: Breathing in air containing pathogens causes them to be trapped in the mucus that lines the lugs. However, some will still reach alveoli in the lungs and cause damage.
• Digestive System: Pathogens wich survive stomach acid can pass into the intestines and invade cells and the gut wall.
• Skin: Pathogens enter the bloodstream through cuts and scratches.

Risk Factors:

• Coronary Heart Disease: 
  • Poor diet (high in salt or saturated fat),
  • Smoking (can lead to high blood pressure),
  • Lack of Exercise (can lead to high blood pressure).

• Cancer:
  • Smoking (can lead to high blood pressure),
  • Excessive sun exposure (can lead to skin cancer).
  • Excessive alcohol intake (can lead to liver cancer).

• A phagocyte is a type of white blood cell (WBC) that carries out phagocytosis.
  • A phagocyte recognises the antigens on a pathogen.
  • The cytoplasm on the phagocyte engulfs the pathogen.
  • The pathogen is now contained in a phagocytic vacuole in the cytoplasm.
  • A lysosome fuses with the vacuole. The lysosomal enzymes digest the pathogen.
  • The phagocyte presents the pathogen's antigens; it sticks the antigens on it's surface to activate the immune system.

• Phagocytes activate T Cells:
  • A T Cell is another type of WBC with proteins on it's surface that bind to antigens.
  • Some release substances to activate B Cells.
  • Some attach to antigens on a pathogen and kill the cell.

• T Cells activate B Cells, which divide into plasma cells.
  • B Cells are also a type of WBC covered in antibodies.
  • Antibodies are proteins that bind to antigens to form antigen-antibody complexes.
    • When the antibody on a B Cell meets a complementary shaped antigen, it binds to it.
    • This activates the B Cell.
    • The activated B Cell divides into plasma cells.

• Plasma cells make more antibodies to a specific antigen.
  • Plasma cells are clones of B Cells. They secrete lots of antibodies.
  • Antibody functions:
    • Coating the pathogen so phagocytes can easily engulf it.
    • Coating the pathogen to prevent it entering host cells.
    • Binding to, and neutralising, toxins produces by the pathogen.

• Cellular Immune Response: T Cells and other cells they interact with.
• Humoral Immune Response: B Cells and antibody production.

• Primary Immune Response:
  • When an antigen first enters the body, it activates the immune system.
  • The primary response is slow due to the lack of B Cells.
  • Eventually, the body produces enough antibodies. The infected person shows symptoms.
  • T&B Cells produce memory cells which remain in the body. 
    • T Cells remember the antigen.
    • B Cells remember the antibody.
  • The person is now immune.

• Secondary Immune Response:
  • Memory B Cells divide into plasma cells.
  • Memory T Cells divide to kill the antigen.
  • The secondary response is quicker and stronger.

• Vaccines contain antigens, causing your body to produce memory cells without getting the disease.
• Herd  Immunity: Vaccines reduce the occurence of the diease, so those not vaccinated are less likely to get the disease.
• Vaccines contain free or dead/weakened pathogens.
• Vaccines may be injected or taken orally.
  • If taken orally, the vaccine could be broken down by enzymes, or the molecules could be too large to be absorbed into the blood.

• Booster vaccines are given later on to ensure that memory cells are produced.
• Antigens on the surface of pathogens activate the primary response.
• The second time of infection, the secondary response is activated.
• Antigenic variability: Pathogens can change their surace antigens. 
• So the second time of infection, the memory cells won't recognise the different antigens; the primary response starts again.
• The primary respose takes time, so you will get ill again.
• Antigenic variation makes it difficult to develop vaccines.

• Monoclonal Antibodies are produced from a single group of genetically identical B Cells. They're all identical in structure.
• Monoclonal antibodies can be made to bind to anything.

• Example: Cancer.
  • Cancer cells have antigens called tumour markers, which aren't found on normal body cells.
  • Monoclonal antibodies are made to bind to tumour markers.
  • You can attach anti-cancer drugs to antibodies.
  • So drugs will only accumulate where there are cancer cells.
  • Side effects of this are loer because they accumulate near specific cells.