AS AQA Unit 1 Biology - Immunity and Disease

Micro-organism is a general term for a single-celled organism that is too small to be seen without a microscope.

Disease may be caused by infectious pathogens or may reflect the effects of lifestyle. Infectious Diseases are communicable diseases which are caused by pathogens: bacteria, viruses, fungi or parasites.                                      

A Pathogen is any organisms that cause disease.

Pathogens include micro-organisms and some larger organisms as well (e.g. Tape worms). Pathogenic micro-organisms include some bacteria, all viruses and some fungi. 

For a micro-organism to be considered a pathogen it must: gain entry to the host, colonise the tissues of the host, resist the defences of the host and cause damage to the host tissues.

If a pathogen gets into the host and colonises it's tissue, an infection results. Disease occurs when an infection leads to recognisable symptoms.

Pathogens need to enter the body to cause disease, they get in through an organism's surface of contact with the environment. There are three main interfaces with the environment: 

-Gas-Exhcange System- breathing in air , most of the pahtogens will get trapped the mucus lining the lung epithelium. These cells also have cilia that beat and move the mucus up the trachea to the mouth, where it is removed. Some pathogens still manage to reach the alveoli though.                                     -Skin- if damage to your skin occurs, pathogens on the surface can enter your bloodstream. The blood clots at the area of damage to prevent pathogens from entering, but some may get in before this.                                                -Digestive System- most pathogens eaten or drunk will be killed by acidic conditions of the stomach. Some may survive and pass through the intestines where they can invade cells of the gut wall and cause disease.

Once inside the body, Pathogens can cause disease by: producing toxins (many bacterial pathogens produce toxins (e.g. Cholera) OR damaging host tissues (pathogens can physically damage host cells by rupturing them to release nutrients, breaking down nutrients in the cell which starves it and replicating in the cells and bursting them.- This is how symptoms are caused.

There are a number of diseases that result from a person's lifestyle.

Risk - is a measure of the probability that damage to health will occur as a result of a given hazard : the probability that the hazard will happen, the consequences of the hazard happening.

Risk factors for cancer in general: smoking, excessive alcohol intake, diet, physical activity.

Risk factors for CHD, smoking, high blood pressure, blood cholesterol, obesity, Diet- high in saturated fats (LDL) or salt, lack of excersize, excessive alcohol intake

Immunity- disease is the interaction between a pathogen and the body's various defence mechanisms. Sometimes the pathogen overwhelms the body and the person dies. Sometimes the bodies defences overwhelm the pathogen and they recover. The body's defences are then better prepared for a second infection from the same pathogen.                                                                            

-Antigens are molecules found on the surface of cells. When a pathogen invades the body the antigens on it's cell surface are identified as foreign, activating cells in the immune system. Foreign antigens trigger an immune response. Antigens are proteins, part of cell membranes/cell walls and trigger the production of antibodies.

-To defend the body from invasion by foreign material, lymphocytes must be able to distinguish the body's own cells and chemicals (self) from those that are foreign (non-self). If they could not do this, the lymphocytes would destroy the organism's own tissues.

Enzymes breakdown pathogens before they get across the interfaces.

Defence Mechanisms- The human body has a range of defences to protect itself from pathogens. There are two main types: 

Specific mechanisms that do not distinguish between one type of pathogen and another, but respond to all of them in the same way. These mechanisms act immediately and take two forms:

-a barrier to the entry of pathogens

- phagocytosis

Non-Specific mechanisms that do distinguish between different pathogens. The responses are less rapid but provide long-lasting immunity. The responses involve a type of white blood call called a lymphocyte and again in two forms:

-cell-mediated responses involving T-lymphocytes. (made in Thymus).

-humoral responses involving B-lymphocytes (made in bone marrow).

Epithelium covered in a Mucus Layer - makes it difficult for pathogens to penetrate. 

Skin- provides a physical barrier that most pathogens find hard to penetrate. Sebaceous glands produce oils which help kill microbes.

Hydrochloric Acid- found in the stomach, proves such low pH so that the enzymes of most pathogens are denatured. 

Nasal Hairs - keeps out dust and larger organisms.

Tears, Saliva, Mucus and Sweat- contain an enzyme called lysozyme which destroys micro-organisms.

Once an individual has experienced a particular species of pathogen it will fight off future infections by the same pathogen much more quickly. The individual is said to have immunity to that pathogen.

A phagocyte (e.g. a macrophage) is a type of white blood cell that carries out phagocytosis (engulfment of pathogens). They're found in the blood and in tissues and are the first cells to respond to a pathogen inside the body.

1. A phagocyte recognises the antigens on a pathogen. Chemicals on the pathogen attract the phagocyte towards it. The Phagocyte attaches itself to the surface of the pathogen.

2. The cytoplasm of the phagocyte moves round the pathogen, engulfing it.

3. The pathogen is now contained in a phagocytic vacuole (a phagosome) in the cytoplasm of the phagocyte.

4. A lysosome (an organelle within the phagocyte that contains lysosomal enzymes) migrates towards the phagosome and fuses with it. The lytic enzymes break down the pathogen.

5. The broken down products are absorbed by the phagocyte. The phagocyte then presents the pathogen's antigens- it sticks the antigens on it's surface to activate other immune system cells.

Cell-Mediated Immunity (cellular)- 

Antigen-presenting cells-   When the body cells are invaded by pathogens the antigens of the pathogen become embedded in the cell membrane. There are three types of antigen –presenting cells: Body cells that have been invaded by pathogens,        Phagocytes have antigens from the pathogens that they have engulfed and        Cancer cells have non self antigens on the surface.

T lymphocyte response- The T lymphocytes only respond to antigens attached to body cells,   Receptors on T helper cells fit onto the antigens (there is a different type of T cell for every different type of antigen) – they are complementary,   Other T cells are activated and divide rapidly by mitosis forming many clones,   The cloned T cells can then do a number of different things;   Some develop into memory cells,   Some stimulate phagocytosis,   Some stimulate B cells to divide,   Some kill infected body cells – they do this by releasing a protein that forms holes in the cell membrane of the cells that have been infected.

Primary Response - Pathogen enters the body and is engulfed by a phagocyte, which presents antigens on it's surface, activating T-lymphocytes.

-T cells in turn activate B cells.

-B cells produce antibodies but this is sometime after the initial symptoms have started.

-T & B cells produce memory cells which remember the antigen that caused the disease.

Secondary Response- Memory B cells divide into plasma cells (by mitosis) which secrete antibodies.

-Memory T cells divide into the different types of T cells so they can destroy the cells that are infected.  

-This response is quicker- greater concentration of antibodies = person does not usually show symptoms.

Humoral Response- Antibodies are transported in the blood and body fluids. Each type of B lymphocyte produces a different type of antibody that responds to one specific type of antigen or toxin (these also act as antigens). Pathogens have many different antigens on their surface.

B lymphocyte response-    Antibody of surface of B cell attach t complementary antigen on the surface of a pathogen in the blood.    This type of B cell then divides by mitosis producing lots of B cells that produce the antibody specific to this antigen.     The cloned B cells then develop into either:

a)      Plasma cells which secrete antibodies producing the primary immune response (only lasts a few days)

b)      Memory cells (these last decades). They don’t produce antibodies but if they come into contact with the antigen again they divide very rapidly, producing plasma cells and more memory cells, which is the secondary immune response.

Antibodies are protein molecules which are synthesised by plasma B cells. They are released in response to a particular antigen and are specific to that antigen. Antibodies neutralise antigens to stop them reproducing and spreading around the body.

Function of Antibodies -  Antigen-antibody complex forms, immobilising pathogens so they cannot move around the body or enter host cells.

- The two antigen binding sites make agglutination (clumping together of pathogens) easier for the antibodies to achieve. This interlocking of antibody and antigen blocks the active region of the antigen, rendering it harmless.

-Agglutination also makes the pathogens easier to track down by the phagocytes.

-The antibody-antigen complex stimulates the production of a plasma protein called complement, which is capable of destroying pathogens, causing inflammation, and coating the pathogen, making it easier for the phagocyte to ingest it.

Two types of immunity: Passive- when you get antibodies from elsewhere. They are short lived / temporary (e.g. Placenta and Breast Milk from Mother, Antiserums -antibodies can be injected into a person as part of medical treatment).

Active- making antibodies for yourself. These are long lasting (e.g. Natural-daily exposure to pathogens, Artificial- Vaccination).

Vaccines- usually contain either the pathogen or the antigens treated so that they cannot cause the disease. Instead, they stimulate the primary response so that when the actual pathogen is encountered the secondary response is strong enough to prevent the disease from developing.

Types of Vaccine- can use: -attenuated pathogens (weakened) - protect against measles, mumps and rubella

- dead pathogens - diphtheria

-purified antigens- these can be made by using genetic engineering techniques.

Successful Vaccination Programmes:    use a cheap vaccine,   use an easily transportable and storable vaccine,   use a vaccine with few side effects,   have many trained staff to administer the vaccine,   vaccinate all vulnerable individuals at the same time.

If for a certain time there are no individuals with the disease then transmission of the pathogen can be interrupted and sometimes eradicated completely.

Vaccinations fail to eradicate all diseases because   some viruses have hundreds or different strains,   some viruses show antigenic variability, vaccinations sometimes fail to produce immunity in certain individuals,    some pathogens can hide from the immune system (e.g. Cholera hides in the digestive system and herpes virus hides in nerves) and    some people can refuse the vaccination.

This is an antibody produced outside the body by fusing a B lymphocyte with a certain type of tumour or cancer cell. The resulting hybrid will divide many times, making large amounts of the required antibody.

Production- A mouse is used as non-self material against which the anti-body is required,    is injected with the antigen,    the mouse makes many different B cells,    the spleen containing the B cells is removed,    detergent is added to help the B cells and cancer cells fuse,    different B cells are cultured to form a group,    each one is tested to see if it is the one needed,   once the one is found it is cloned to produce large amounts of required antibody.                              -Uses: to separate a chemical from a mixture, used in immunoassay (a method for calculating the amount of substance in a mixture), cancer treatment(the antibodies are used to carry the cytotoxic drugs to the cancer cells) and transplant surgery (monoclonal antibodies can reduce the chance of rejection by disarming T-cells).

Arguments For: -used to treat cancer and diabetes. Arguments Against:    animal rights supporters are against it,   transgenic mice (contains human genes) reduce the need for humanisation of the antibodies, but those against generic engineering my object,    these antibodies have caused multiple organ failure in drug trials.