M. Hourigan 2014
The defence system in humans
The human body has 2 defence systems to protect itself from foreign bodies and cells:
 General defence system
 Specific defence system.
General defence system consists of:
1st Line of defence - Barrier to entry:
 the skin
 mucous membrane lining of the respiratory, digestive and reproductive tract.
How they work
 The skin,
 acting as a structural barrier,
 secretes chemicals from the sebaceous glands that harm
or kill bacteria.

The respiratory and digestive tracts as well as other organs of the body secrete mucus
and/or further chemicals that may remove foreign particles. (Acid in stomach kills micro-organisms).
2nd Line of defence - Phagocytic white blood cells.
If foreign bodies get through 1st line of defence then phagocytic white blood cells as follows:
 Some types engulf bacteria and viruses upon contact.
 Others secrete chemicals that stimulate general defence and cause fever to destroy microbes at high
temperatures.
Specific defence system
The specific defence system attacks specific pathogens.
Organs specific to the immune system:
 the spleen,
 thymus
 and lymph nodes.
Lymph and blood vessels contain white blood cells called lymphocytes and monocytes.
Lymphocytes and monocytes are produced in the bone marrow.
These cells respond to antigens.
Antigens are defined as foreign molecules capable of eliciting an antibody response.
Antigens may be found in
 bacterial cell walls,
 viral coats,
 foreign cells
 or produced in cancerous cells.
Antigen immunity usually lasts for a long time.
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M. Hourigan 2014
Antibody – a protein produced by white blood cells (lymphocytes) in responxe to an antigen.
(see book for how they work p 380).
Antigen – antibody reactions are very specific i.e. only one type of antibody will fit one antigen.
Immunity = ability to resist infection.
Induced immunity
Induced immunity = ability to resist disease caused by specific pathogens.
Induced immunity involves antibodies
There are two types of induced immunity
1. active immunity
2. passive immunity.
Active immunity
Active immunity involves the body making its own antibodies in response to antigens (on the pathogens)
that enter the body.
Active immunity:
 develops after vaccination or after an infection
 provides long lasting protection.
Active immunity may be induced in 2 ways:
1. Natural active immunity occurs by suffering a disease )e.g. cold, flu, chicken pox etc.)
2. Artificial active immunity acquired by injection – vaccination / immunisation.
Vaccine = a sample of the pathogen that has been treated so it doesn’t reproduce but still has the antigens
necessary to produce antibodies.
Passive immunity
 occurs when individuals are given antibodies formed in another organism to combat a disease,
 since these are not produced by the body’s cells, they are short lived.
N.B. Passive immunity is short lived. In passive immunity, the body does not make the antibodies involved.
Types of Passive Immunity
1. Natural Passive Immunity
Acquired by a baby from its mother – antibodies pass from mother through the placenta to the baby
or
antibodies passed through breast milk to baby.
2. Artificial Passive Immunity
Acquired by injection – antibodies made in another organism are injected into the person to fight the
micro-organism e.g. tetanus antibodies injected to fight tetanus (lock jaw).
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M. Hourigan 2014
Role of lymphocytes:
There are two types of lymphocyte cells:
 B cells
 and T cells.
B cells produce antibodies – proteins capable of combining with and inactivating
antigens by surface recognition. Each B cell produces just one type of antibody.
T cells do not produce antibodies instead they act in one of four processes:

as helper T cells which recognise antigens, enlarge, and secrete chemicals, such as
interferon, which stimulate the production of B cells

as killer T cells which attack cells containing a foreign antigen, secrete a chemical
called perforin that perforates the membranes of cells

as suppressor T cells which stop immune responses

as memory T cells which can memorise the immunity, even for life.
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