Biology- Unit 1 - Topic 6

HideShow resource information
  • Created by: Rosa
  • Created on: 05-05-13 11:43


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
1 of 9


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 & LYMPHOCYTESPhagocytes 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 
2 of 9

T cells and cell-mediated immunity

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) 
3 of 9

Cell-mediated Immunity

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 

4 of 9

B cells and humoral immunity

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 
5 of 9

B cells and humoral immunity

  • 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 

6 of 9


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 
7 of 9


  • 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 
8 of 9


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

9 of 9


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all resources »