The Defence System

Chapter 38

Pathogens

Pathogens are disease causing organisms

The human body has 2 ways of defending against pathogens

1. The GENERAL defence system

2. The SPECIFIC defence system

The General Defence System

Acts as a barrier to pathogens entering the body

Barrier Reason

Skin Secretes enzymes from the sebaceous glands that kill bacteria. It is a physical barrier

Mucous membrane

Lines breathing, reproductive & digestive tracts, Its sticky & traps pathogens before they enter the body

Phagocytic WBC Ingest pathogens

Specific Defence System

This system is activated when pathogens get past the general defence system

Organs of the immune system that store WBC’s called lymphocytes and monocytes include the lymphatic vessels, tonsils, spleen lymph nodes

Antigens

An antigen is a foreign molecule that causes antibody production

Antigens are located on:

bacterial cell walls

viral coats

cancerous cells

Antibodies

An antibody is a protein produced by white blood cells in response to an antigen

Each type of antibody is highly specific to a single antigen

Antibodies inactivate antigens and allow them to be destroyed

Monocytes & LymphocytesMONOCYTES:Are WBC formed in the bone marrowThey engulf foreign bodiesThey move into the lymphatic system eg lymph

nodes, vessels ,spleen & thymus glandLYMPHOCYTES:WBC formed in the bone marrowMove into lymphatic system--- produce

antibodies2 types – B cells & T cells

Advanced Study of Lymphocytes Higher Level Only

B – lymphocytes (B-cells)Become active in the bone marrowEach B cell produces only one type of

antibodyWhen a B cell comes in contact with an

antigen it divides into thousands of identical B cells ( called plasma cells)

These plasma cells produce antibodies that combine with and deactivate the antigen

THE IMMUNE RESPONSE

Plasma cells only live for a few days and most die off once infection is over

Some live on forever , these are called memory B –cells, if the same antigen enters the body again these B- cells produce more antibodies much faster than before, which prevents us being infected with the same antigen more than once

T- lymphocytes ( T-cells)

Become active in the thymus gland Do not produce antibodies Act in one of 4 ways

1. Helper T-cells

2. Killer T- cells

3. Suppressor T- cells

4. Memory T-cells

Helper T- cells

Recognise antigens from other white blood cells

Enlarge and form clonesThese stimulate the production of B -cells

Killer T-cells

Destroy abnormal ( infected ) body cellsRecognise antigens on the surface of

infected cellsThey release proteins ( perforin) which

form pores in the membrane of target cellsWater & ions flow in through these poresInfected cells swell & burst

Suppressor T-cells

Control ( suppress) the immune response

Memory CellsMemory T- cells survive for life and

memorise the immune response

Induced Immunityhigher & ordinary level

Is the ability to resist disease caused by infection

There are 2 types of induced immunity

1. Active immunity

2. Passive immunity

Active Immunity

Means the person produces his/her own antibodies in response to antigens

It is long term immunityIt develops after a vaccination or an

infection

(artificial) (natural)

Getting a vaccine

Pathogens are introduced into the body

Pathogens entering body in a natural manner eg getting a cold

Passive Immunity

Occurs when people are given antibodies to fight disease

They are not made by the person’s own immune system

It is short term immunity---eg. child getting antibodies naturally from its mother’s placenta or breastmilk (natural) OR getting and injection of foreign antibodies eg tetanus injection(artificial)

Vaccination

Is an injection of a killed pathogen in order to stimulate the immune system against the pathogen, thereby preventing the disease being suffered Eg. MMR, BCG

IMMUNISATION

Is a process that increases an organisms reaction to an antigen & therefore improves its ability to resist or overcome infection