diseases and immunity Flashcards
A pathogen is a
disease-causing organism
transmissible diseases because
Pathogens are passed on from one host to another
Pathogens can be passed on from host to host in different ways, including:
Direct contact – the pathogen is passed directly from one host to another by transfer of body fluids such as blood or semen (eg HIV, gonorrhoea, hepatitis B & C)
Indirect contact – the pathogen leaves the host and is carried in some way to another, uninfected individual
There are three main ways in which the body defends itself against disease:
- Mechanical barriers – structures that make it difficult for pathogens to get past them and into the body
a) Skin – covers almost all parts of your body to prevent infection from pathogens. If it is cut or grazed, it immediately begins to heal itself, often by forming a scab.
b) Hairs in the nose – these make it difficult for pathogens to get past them further up the nose so they are not inhaled into the lungs
- Chemical barriers – substances produced by the body cells that trap / kill pathogens before they can get further into the body and cause disease
a) Mucus – made in various places in the body, pathogens get trapped in the mucus and can then be removed from the body (by coughing, blowing the nose, swallowing etc)
b) Stomach acid – contains hydrochloric acid which is strong enough to kill any pathogens that have been caught in mucus in the airways and then swallowed or have been consumed in food or water
- Cells – different types of white blood cell work to prevent pathogens reaching areas of the body they can replicate in
a) By phagocytosis – engulfing and digesting pathogenic cells
b) By producing antibodies – which clump pathogenic cells together so they can’t move as easily (known as agglutination) and releasing chemicals that signal to other cells that they must be destroyed
Antigens & Antibodies
All cells have proteins and other substances projecting from their cell membrane
These are known as antigens and are specific to that type of cell
Lymphocytes have the ability to ‘read’ the antigens on the surfaces of cells and recognise any that are foreign
They then make antibodies which are a complementary shape to the antigens on the surface of the pathogenic cell
The antibodies attach to the antigens and cause agglutination (clumping together)
This means the pathogenic cells cannot move very easily
At the same time, chemicals are released that signal to phagocytes that there are cells present that need to be destroyed
The initial response of a lymphocyte encountering a pathogen for the first time and making specific antibodies for its antigens can take a few days, during which time an individual may get sick
Lymphocytes that have made antibodies for a specific pathogen for the first time will then make ‘memory cells’ that retain the instructions for making those specific antibodies for that type of pathogen
This means that, in the case of reinfection by the same type of pathogen, antibodies can very quickly be made in greater quantities and the pathogens destroyed before they are able to multiply and cause illness
This is how people can become immune to certain diseases after only having them once
It does not work with all disease-causing microorganisms as some of them mutate fairly quickly and change the antigens on their cell surfaces
Therefore, if they invade the body for a second time, the memory cells made in the first infection will not recall them as they now have slightly different antigens on their surfaces (e.g. the cold virus)
Active Immunity
Making antibodies and developing memory cells for future response to infection is known as active immunity
There are two ways in which this active immune response happens:
The body has become infected with a pathogen and so the lymphocytes go through the process of making antibodies specific to that pathogen
Vaccination
Active immunity is slow-acting and provides long-lasting immunity
Passive Immunity
This is when ready-made antibodies, from another source, are introduced to the body
Passive immunity is a fast-acting, short-term defence against a pathogen by antibodies acquired from another individual, eg:
From mother to infant via breast milk – this is important as it helps the very young to fight off infections until they are older and stronger and their immune system is more responsive
Injected antibodies for certain diseases where the individual is already infected and a fast response is required, like rabies or tetanus
The body does not make its own antibodies or memory cells in passive immunity, hence the name
Diseases Caused by the Immune System
Occasionally, the cells of the immune system start to attack the body’s own cells
This is rare as lymphocytes usually recognise their own body cells by the antigens on the cell surfaces and do not respond to them
In this situation, specific body cells are targeted by lymphocytes and antibodies are made against them, destroying them
One example of this type of disease is Type 1 diabetes
People who suffer from this disease no longer make their own insulin and so are unable to regulate their blood glucose levels
This is because their immune system is targeting and destroying the pancreatic cells which are responsible for making the insulin, eventually leading to dangerously high glucose levels in the blood
An antigen is
a chemical found on the surface of a cell
An antibody is
a chemical made by lymphocytes that is complementary to an antigen and, when attached, clumps them together and signals the cells they are on for destruction
An antibiotic is
a drug that slows down or stops the growth of bacteria
What is Vaccination?
Vaccinations give protection against specific diseases and boost the body’s defence against infection from pathogens without the need to be exposed to dangerous diseases that can lead to death
The level of protection in a population depends on the proportion of people vaccinated
How does Vaccination Work?
Vaccines allow a dead or altered form of the disease-causing pathogen, which contains specific antigens, to be introduced into the body
In this weakened state, the pathogen cannot cause illness but can provoke an immune response
Lymphocytes produce complementary antibodies for the antigens
The antibodies target the antigen and attach themselves to it in order to create memory cells
The memory cells remain in the blood and will quickly respond to the antigen if it is encountered again in an infection by a ‘live’ pathogen
As memory cells have been produced, this immunity is long-lasting
How does Vaccination Control the Spread of Disease?
If a large enough percentage of the population is vaccinated, it provides protection for the entire population because there are very few places for the pathogen to breed – it can only do so if it enters the body of an unvaccinated person
This is known as herd immunity
If the number of people vaccinated against a specific disease drops in a population, it leaves the rest of the population at risk of mass infection, as they are more likely to come across people who are infected and contagious
This increases the number of infections, as well as the number of people who could die from a specific infectious disease
This is the reason that many vaccinations are given to children, as they are regularly seen by medical practitioners and can be vaccinated early to ensure the entire vaccinated population remains at a high level
In certain instances, vaccination programmes are run with the aim of eradicating certain dangerous diseases, as opposed to controlling them at low levels
An example of a disease which has been eradicated as a result of a successful vaccination programme is smallpox, which was officially eradicated in 1980 after a vaccination programme run by the World Health Organisation since the mid-1950s
Ways to Prevent Transfer of Pathogens
