On the surface of all cells are chemical markers (for example, proteins) called antigens.
Your body recognises the antigens on your cells as your own (self); anything with different antigens to you (non-self) stimulates an immune response.
In an immune response, your body will recognise the antigen as foreign (and therefore bad) and will attack it.
Your immune system is made up of cells that work with the body's physical and chemical barriers. It helps prevent any pathogen (disease-causing organism) entering your body, and your body therefore becoming infected.
Note: Harmful bacteria is an example of a pathogen.
If the worst comes to the worst and any pathogens do get into your body, the immune system tries to stop them from causing harm.
The first line of defence is made up of physical and chemical barriers.
These types of barriers are non-specific (i.e. if any organism is not recognised, it is assumed to be a pathogen, and will be treated the same way).
These barriers occur at the skin or any other openings to the outside world.
Here is a list of some physical and chemical barriers you should know to quote in your exams:
Skin: it is a hard outer layer that generally prevents the entry of any undesirables.
Nose, throat and digestive tract: the membrane lining these secretes sticky mucus to trap microbes. Fine hairs called cilia waft the mucus away.
Eyes: tears have lysozyme enzyme in them. This kills some bacteria.
Ear: your wax has antimicrobial properties.
Stomach: hydrochloric acid in your stomach kills bacteria.
Large intestine, urethra and vagina: resident harmless bacteria use the nutrients that any harmful microbes would need to survive (i.e. harmless bacteria out-compete the harmful bacteria).
Sweat: is an acidic liquid that contains enzymes which kills some bacteria.
The second line of defence is also a non-specific response (i.e. the response is the same for any pathogen).
It is a 3-pronged attack on any microbes that have survived the first line of defence...
Inflammation happens because cells damaged by invading pathogens and particular white blood cells release 'alarm' chemicals which makes blood vessels enlarge (vasodilate) and the capillaries more 'leaky'.
Inflammation attracts white blood cells to the area.
The three types of white blood cell you need to know for your exam are neutrophils, macrophages (these are both phagocytes, which are engulfing cells), and lymphocytes.
The phagocytes (for example a neutrophil), having squeezed through the capillary wall and into the infected tissue, engulf and digest offending bacteria as shown in the following diagram...
Other than direct 'hand-to-hand' combat, some killing is done at a distance.
Macrophages make proteins that act in two ways:
- They can punch holes in the bacteria and parasites so that they die.
- Or the proteins can stick to the outside of the bacteria to make them more appealing for the phagocytes to eat!
If a virus or an intracellular parasite (one that lives inside a cell) has invaded a cell, the cell will make a chemical called interferon. Interferon ultimately prevents that cell from making molecules that the pathogen would need to survive.
The third line of defence depends on lymphocytes. There are two basic types of lymphocyte and both are made in bone marrow.
T Cells: mature after having first migrated from the bone marrow to the thymus gland. Involved in the cell-mediated response
B Cells: migrate to and then mature in either the bone marrow or in the foetal liver or spleen. Involved in the humoral response.
If an antigen is presented to a T cell with a complementary shaped receptor, the T cell is stimulated, increases in size and starts to divide.
A clone of identical T cells is formed, all with the correct shaped receptor. These T cells then differentiate to form 4 groups of specialised T cells. These are:
Killer T cells
Helper T cells
Suppressor T cells
As with T cells, a B cell will form a clone if it comes into contact with a complementary shaped antigen. The clone contains mostly plasma cells for immediate use and some memory cells for use in the future.
The plasma cells are highly developed and are able to make several thousand antibody molecules every second.
Plasma cells and most of T cells die after only a few days. However, the memory B cells and a few memory T cells survive.
Each plasma cell and T cell will only be programmed to only respond to the one antigen that they have already encountered. So they wait in the lymph nodes in case re-infection occurs, in which case they are ready to attack.
This way, although the first infection was dealt with in a few days to a few weeks by the primary response, the secondary response to re-infection is much quicker and much more powerful.
Vaccine is small quantities of the antigen attached to the offending organism.
If the activation of the immune system occurs naturally during an infection, this is termed natural immunity. Because in response to the antigens, Band T cells have gone into action and a memory has been produced, it is also termed active immunity. Vaccination would also be termed a form of active immunity.
These are drugs used to treat or cure infections and to be effective they must kill or disable the pathogen, leaving host cells unharmed. Most antibiotics are used to treat bacterial and fungal infections, there are very few that are effective against viruses. A few antibiotics are synthetic but most are derived from living organisms. They work by either interfering with the growth or metabolism of the bacteria or fungi. They may inhibit the synthesis of the cell wall, translation or transcription of proteins, interfere with membrane function or enzyme action.
Penicillins are well known antibiotics, which work by preventing the synthesis of peptidoglycan polymer cross links in the cell walls of bacteria. They are only effective when the organism is making new cell walls, i.e. growing.
These are a result of an overreaction of the immune system to a harmless antigen, as in asthma, hay fever and eczema.
They are caused by allergens - for example, pollen, dust, particles of animal skin, dustmites and their faeces.