10) Diseases and immunity

10) Diseases and immunity


Pathogen: is a disease-causing organism.

Transmissible disease: is a disease in which the pathogen can be passed from one host to another.


Pathogens responsible for transmissible diseases can be spread either through direct contact, eg. through blood or other body fluids, or indirectly,, eg. from contaminated surfaces or food, from animals, or from the air.


Defences against disease:

Mechanical barriers – skin and hair in the nose.

Chemical barriers – stomach acid, mucus produced by the lining of the trachea and bronchi, and tears which contain an enzyme called lysozyme.

Cells – phagocytosis and antibody production by white blood cells.

Vaccination – can enhance the body’s defense.


Antibodies and immunity:

  • On the surface of all cells there are chemical substances called antigens.
  • Lymphocytes produce proteins called antibodies which attack the antigens of bacteria that invade the body.
  • The antibodies may attach to the surface of the bacteria to mark them, making it easier for the phagocytes to find and ingest them.
  • Each pathogen has its own antigens, which have specific shapes, so specific antibodies which fit the specific shapes of the antigens are needed.


Active immunity: is the defence against a pathogen by antibody production in the body. This is gained after an infection by a pathogen, or by vaccination.



  1. Inoculated (vaccinated) by harmless pathogen which has antigens.
  2. The antigens trigger an immune response by lymphocytes which produces antibodies.
  3. Memory cells are produced that give long-term immunity.


Systemic immunisation can protect whole populations.


Passive immunity:

  • Is a short-term defence against a pathogen by antibodies acquired from another individual.
  • This is temporary as no memory cells are formed.
  • A baby’s immune responses are not yet fully developed, so when a mother breastfeeds her baby, the milk which contains the mother’s white blood cells produces antibodies and provide the baby with protection against infection.


Type 1 diabetes:

  • Also known as juvenile-onset diabetes.
  • Due to the inability of islet cells in the pancreas to produce sufficient insulin.
  • A virus infection can cause the body’s immune system to attack the islet cells that produce insulin. This is classed as an autoimmune
  • The outcome is that the patient’s blood is deficient in insulin and he/she needs regular injections of the hormone in order to control blood sugar levels and so lead a normal life.