Biology- Unit 1 - Topic 6

Defencre Mechanisms: 

• Non-specific = mechanism that does not distinguish between one type of pathogen and another , but respond to all of them in the same way - they act immediatley - and take 2 forms - 1. a barrier to the entry of pathogens 2. phagocytosis
• Specific= mechanisms that DO distinguish between differrent pathoagen- they are less rapid but provide LONG-LASTING immunity- the response involve a type of white cell called a LYMPHOCYTE and take 2 forms - 1. cell-mediated responses involving T LYMPHOCYTES - 2. humoral response involving B LYMPHOCYES 

Recognising your own cells-

• Lymphocytes must be able to distinguish the bodys own cells and chemicals from those that are FOREIGN - if they couldnt they would destroy owns tissue
• Lymphocytes are not produced in response to infection- ALREADY EXISIT - 10 MILLION
• When a pathogen gets into body , 1 of the lymphocytes will have proteins on its surface that is complemntary to a protein on pathogen - RECOGNIZE IT 
• Infection occurs lymphocyte which has complemntary proteins to pathogen - will build up its numbers to destroy it - this is why it takes time to overcome infection

The bodys first line of defence is to form a physical or chemical barrier to enrty , should this fail the next line of defence is the white blood cells- 2 types PHAGOCYTES & LYMPHOCYTES. Phagocytes ingest and destroy and the pathogen by PHAGOCYTOSIS. 

• Barriers to Enrty -   Proctective Covering - skin covers the body surface - physical barrier - pathogens find it hard to penetrate 
• Epithelia covered in mucus- epithlial layers produce mucus which acts as a further defence - in the lungs pathogens stick to this mucus which is then transported away to cilia up the trachea to be swallowed into the stomach
• Hydrochloric acid in the stomach - such a low pH that enzymes of most pathogens are denatured and therefore organisms killed

Phagocytosis- large particles e.g. bacteria are too big to cross cell-surface membranes by diffusion or active transport - instead they are engulfedby cells in the form of veisicles . In the blood the types of white blood cells that carry out phagocytosis are known as phagocytes - provide vital defence against pathogens

• chemical products of pathogen attract causing phagocytes to move towards the pathogen 
• phagocytes attactch themselves to the surface of the pathogen- they engulf pathogen to form a veiscle
• Lysosome move towards veisicles (phagosome) - and fuse with it - enzymes within lysome break down pathogen  into small soulble molecules they are then absorbed into cytoplasm of the phagocyte 

Immunity is the ability of organisms to resist infection by protecting against disease-causing microogransims that invade their bodies 

Antigens- ANY part of an organism or substance that is recognized as non-self FOREIGN by the immune system and stimulates an immune response- they are usually proteins on cell-surface membranes or cell wall of invading cells e.g cancer cells- antigen triggers the production of an antibody as part of the bodys defence system

Lymphocytes- the specific immune response depends on a type of white blood cell called a lymphocyte - there are two of lymphocyte : 

• B lymphocytes (B cells) - are associated with HUMORAL immunity e.g. immunity involving antibodies which are present in body fluids (Mature in Bone Marrow)
• T lymphocytes (T cells) - are associated with cell  mediaited immunity e.g. involving body cells (mature in Thymus gland) 

T lymphocytes respond to an organisms own cell that have been invaded by non-self material and also respond to transplanted material which is genetically different. They can distinguish these invader cells from normal cells as phagoctes that have engulfed and broken down a pathogen , body cells invaded by a virus & cancer cells all present antigens on their surface membrane - they are called antigen-presenting cells

T lymphocyte cells will only respond to antigens which are attached to a body cell so is called cell-mediated immunity- Proccess explained below:

• Pathogens invade body cells or are taken in by Phagocytes
• The phagocyte places antigens from the pathogen on its cell surface-membrane
• receptors on t-helper cells fit exactly onto these antigens
• this activitates other t-cells to divide rapidly by mitosis and form a clone
• The clone T cells - 1. develop into memory cells and enable a rapid response to future infections by the same pathogens.2. stimulate phagocytes to engulf pathogens by phagocytosis.3.stimulate B cells to divide.4.kill infected cells

T-cells kill infected cells as they produce a protein which makes holes in the cell-surface membrane - holes mean cell becomes permeable to all substances 

Some T-cells produce factors that stimulate B cells to divide , these B cells are involved in the next phase of a specific response - humoral immunity. It involves antibodies which are soluble in the blood and tissue-fluid of the body(body fluid - humour). Each B cell produces a different anti-body that responds to one specific anti-gen. e.g. when an anti-gen (protein) enters the blood there will be one B cell that has an antibody which fits exactly to the anti-gen- they are complementary - divide by mitosis and form clones , for each clone , the cells develop into two types of cell:

• Plasma Cells- secrete antibodies - survive for only a few days but can make around 2000 antibodies every second - antibodies produced destroy pathogen and anytoxins  it produces - they are responsible for the immediate defence of the body against infection known as PRIMARY IMMUNE RESPONSE
• Memory Cells - live longer then plasma cells (decades) -do not produce anti-bodies directly but go in the blood and tisue fluid when they encouter same antigen later date they develop into plasma cells and more memory cells.- plama cells then produce antibodies needed to destroy pathogen- while new memory cells circulate for any future infection- they therefore provide long-term immunity against original infection- 'secondary immune response' - rapid and greater intensity - makes sure new infection causes no harm- people unaware 

• The surface antigens of the invading pathogen are taken up by B cells
• B cells process the antigens and present them on their surface
• T-helper cells attach to the antigens on the B cells which activates them
• B cells are now activated and can divide by MITOSIS to give a clone of plasma cells
• Cloned plasma cells produce antibodies that exactly fit the antigens on pathogen surface
• Antiboides attach to antigens on the pathogen ans destroys them 
• However somrtimes B cells develop into memory cells - which can respond to future infections by the same pathogen by dividing rapidly and developping into plasma cells that then produce antibodies (secondary immune response) 

Antigenic Variability - influenza viruses and other pathogens have many different stains , the antigens that these viruses are made of , and those that they produce are constantly changing - the body therefore has to react as if the infection was a new one (primary response) and therefore is slower to react 

They are proteins synthesised by B cells. B cells produce them when body is invaded by foreign material. Antibodies bind with antigens on surface of foreign material. They are very specific each has its own anti-body. Made up of 4 poly-peptide chains, one pair chain is long and called heavy chains and other pair is shorter called light chains.To help anti-body to fit around anti-gen they can change shape (moving as if they had a hinge at the fork of a Y-shape). They have a binding site that fits precisley to antigen called 'antigen-antibody complex'.  Binding site is different on each anti-body and therefore is called the variable region. Each site is made up of a sequence of amino acids to form a specific 3D shape that binds directly to antigen. Rest of the antibody is the same in all antibodies and is called the constant region

Monoclonal antibodies - it is very important medically to be able to produce antibodies outside the body , its even better if a single type of antibody can be isolated and cloned, they have a number of useful functions:

• seperation of chemicals from a mixture
• cancer treatment - can be made to only attach themselves to cancer cells- they can be used to activate a cytoxic drug  (1 that kills cells)- only be activated by cells to which the monoclonal antibosy is attached - e,g destroy cancer cells 
• immunoassay - method to calculating how much substance in a mixture e.g. testing for drugs in athletes urine & pregnacy tests 

• Passive immunity - an introduction on antibodies into individuals from an outside source- as antibodies not being produced by individuals themselves they are not replaced when broken down in body - immunity is short-lived
• Active immunity - stimulating the production of antibodies by the individuals own immune system - longlasting

Vaccination - introduction of a substance in the body with the intention of stimulating active immunity against a particular disease- on a large scale protection for whole populations 

Features of a successful vaccination programme:

• Must be economically avaliable in sufficent quantaties to immunise all of vunreable poulation 
• must be very few side-effects , if any. Unpleasant side-effects discourage people from the vaccine
• producing , storing and transporting vaccine must be avaliable - technologically advane equipment
• training staff with approriate skills throughout the population 
• To vaccinate vast majority of vunreable population - best done at one time so no indivduals have the disease in the population and the transmission of pathogen is interrupted- HERD IMMUNITY 

Why Vaccination does not elimnate a disease: 

• it fails to induce immunity in certain individuals e.g peopel with defective immune systems
• individuals may develop disease straight after vaccination - immunity levels not high enough to prevent it
• pathogen may mutate frequently - antigens change suddenly - vaccine ineffective- antigens on pathogen are not recognized by immune system - no antibodies produced to destroy pathogen
• many varieties of pathogens - impossible to develop a vaccine against them all 
• certain pathogens hide from bodys immune system - concealing themselves in other cells or live in places out of reach such as within intestines e.g. cholera pathogen 
• Individuals have objection to vaccine e,g religious or ethical reasons 

Control of Cholera by means of vaccination is difficult because : 

• is an intestial disease and not easily reached by immune system-
• Antigens of cholera pathogen change rapidly - hard to decelop effective vaccine
• Cholera spreads rapidly e.g. tourism- difficult to ensure people are vaccinated 

Control of Tb by vaccination is difficult because  - HIV infection led to more people with impaired immune systems- more likley to contact TB. Poverty , wars means many refugees who move around frequently and are in overcrowded housing - tourism and fleeing wars spread disease worldwide - more old people weak immun