DISEASES AND IMMUNITY Flashcards
Pathogen
A disease-causing organism
Transmissible disease
A disease in which the pathogen can be passed from one host to another
Active immunity
The defence against a pathogen by antibody production in the body
Direct contact may involve transfer through blood or other body fluids.
- For example, HIV can be passed on by drug addicts who inject the drug into their bloodstream and share needles with other drug users, as the needle will be contaminated.
- Anyone cleaning up dirty needles is at risk of infection if they accidently stab themselves.
- Surgeons carrying out operations have to be especially careful not to be in direct contact with the patient’s blood.
- A person with HIV or another sexually transmitted disease (see Chapters 15 and 16) who has unprotected sex can pass on the pathogen to their partner through body fluids.
ndirect contact may involve infection from pathogens on contaminated surfaces, for example during food preparation
- Raw meat carries bacteria, which are killed if the meat is adequately cooked. However, if the raw meat is prepared on a surface that is then used for other food preparation, e.g. cutting up vegetables that are later eaten raw, the pathogens from the meat can be transferred to the fresh food.
- People handling food are also potential vectors of disease, e.g. if they do not wash their hands after using the toilet.
- Intensive methods of animal rearing may contribute to the spread of infection unless care is taken to reduce the exposure of animals to infected faeces.
- Air-borne infections can be spread by a person with an infection sneezing or coughing. Droplets containing the pathogen float in the air and may be breathed in by other people or fall on to exposed food. Examples of diseases spread in this way include colds, influenza (flu), measles and sore throats.
• Mechanical barriers
- The epidermis of the skin is a barrier that prevents bacteria getting into the body.
- Hairs in the nose help to filter out bacteria that are breathed in.
• Chemical barriers
- The acid conditions created by hydrochloric acid in the stomach destroy most of the bacteria that may be taken in with food.
- Mucus, produced by the lining of the trachea and bronchi, is sticky and traps pathogens.
- Tears contain an enzyme called lysozyme. This dissolves the cell walls of some bacteria, protecting the eyes from infection.
• Cells
- One type of white blood cell produces antibodies that attach themselves to bacteria, making it easier for other white blood cells to engulf them.
- Another type of white blood cell engulfs bacteria (a process called phagocytosis) and digests them (see
Vaccinations
Vaccination gives a person immunity to a specific disease organism, which may otherwise be life threatening if a person is infected by it.
Methods of controlling the spread of disease
- Hygienic food preparation: keeping food-preparation surfaces clean, avoiding the preparation of raw and cooked food on the same surface, cooking food thoroughly to kill any bacteria present.
- Good personal hygiene: washing hands after using the toilet, moving rubbish or handling raw food; avoiding the handling of money when handling unwrapped food. • Waste disposal: to avoid the development of a breeding ground for pathogens.
- Sewage treatment: to prevent pathogens in faeces from contaminating drinking water and to stop vectors such as flies or rats feeding and transmitting the disease organism.
Action of antibodies
Antibodies are produced by lymphocytes, formed in lymph nodes in response to the presence of pathogens such as bacteria. The pathogens have chemicals called antigens on their surface; there is a different antigen with a specific shape for each type of pathogen. So, a different antibody with a matching shape has to be produced for each antigen.
The antibodies make bacteria clump together and mark them, in preparation for destruction by phagocytes, or neutralise the toxins produced by the bacteria. Once antibodies have been made, they remain in the blood to provide long-term protection. Some lymphocytes memorise the antigens the body has been exposed to. They can rapidly reproduce and produce antibodies to respond to further infections by the same pathogen.
Active immunity
Some of the lymphocytes that produced the specific antibodies remain in the lymph nodes for some time and divide rapidly and make more antibodies if the same antigen gets into the body again. This creates immunity to the disease caused by the antigen. Active immunity can also be gained by vaccination.
The process of vaccination
- Vaccination involves a harmless form of the pathogen that has antigens being introduced into the body by injection or swallowing.
- The presence of the antigens triggers lymphocytes to make specific antibodies to combat possible infection. • Some of these cells remain in the lymph nodes as memory cells.
- These can reproduce fast and produce antibodies in response to any subsequent invasion of the body by the same pathogen, providing long-term immunity.
- Mass vaccination can control the spread of diseases. There needs to be a significant proportion of a population immunised to prevent an epidemic of a disease, ideally over 90%. If mass vaccination fails, the population is at risk of infection with the potential for epidemics.
Passive immunity is a short-term defence against a pathogen.
- It is achieved by injecting the patient with serum taken from a person who has recovered from the disease. Serum is plasma with the fibrinogen removed and contains antibodies against the disease, e.g. tetanus, chickenpox, rabies.
- It is called passive immunity because the antibodies have not been produced by the patient. It is only temporary because it does not result in the formation of memory cells.
- When a mother breastfeeds her baby, the milk contains some of the mother’s lymphocytes, which produce antibodies.
- These antibodies provide the baby with protection against infection at a time when the baby’s immune responses are not yet fully developed. However, this is another case of passive immunity, as it is only short-term protection: memory cells are not produced.
Autoimmune diseases
Type 1 diabetes mainly affects young people. It is due to the inability of islet cells in the pancreas to produce sufficient insulin. It may be inherited, but can be triggered by an event such as a virus infection. This causes the body’s immune system to attack the cells in the pancreas that produce insulin. It is therefore classed as an autoimmune disease. As a result, the patient’s blood is deficient in insulin and regular injections of the hormone are needed to control blood sugar levels and allow the patient to lead a normal life (see Chapter 14).
