1. Internal Defence
The Immune System
Yr 11 Biology

2. Internal Defence
Animals have internal defence systems to
protect them against disease-causing organisms
that enter the body via;
•Air
•Food
•Water
•Wounds in the skin

3. What causes disease?
Disease-causing microorganisms include:
•Viruses
•Bacteria
•Fungi
•Protozoa
All of these organisms are collectively known as
pathogens.

4. Self from non-self?
Internal defence relies on the animal’s ability to
recognise “self” from “non-self”.
Possible because:
•Organisms are biochemically unique
•Cells have specific surface proteins
•Identify between members of the same species
•The organism “knows” own cells, and identifies
other cells as “foreign”

5. The invasion - Antigens
During an invasion, the presence of these
“foreign” cells stimulate a defensive response.
A substance capable of stimulating an immune
response is called an antigen.
Foreign macromolecules include proteins, RNA,
DNA and some carbohydrates are all antigenic.

6. The Defence Mechanisms
There are 2 types of defence mechanisms:
1.Nonspecific defence – provide general protection
against pathogens. Prevent entry, rapidly destroy
cells that penetrate the body.
2.Specific defence – tailor-made to combat specific
antigens associated with each pathogen. These are
collectively known as the immune response.
Includes antibody production (specific proteins that
bind to antigens).

7. Defence Memory
Specific defence also has an immunological
memory – the capacity to respond more
effectively the 2nd time a foreign molecule
invades the body.

8. Nonspecific Defence
1st line – outer covering or skin
•A mechanical and chemical barrier to
microorganisms.
•Sweat and sebum (fats and oils secreted by the
skin).
•Contain chemicals that destroy certain types of
bacteria.

9. 2nd line – internal fluids and structures
•HCl and enzymes in stomach – destroy
microbes that enter with food
•Cilia in respiratory tract – filter pathogens in air
•Mucus lining – traps pathogens (which are then
“eaten”
•Lysozymes – enzymes in tissues, tears and body
fluids, attack cell walls of some bacteria

10. 3rd line – the damaged cells.
•Release cytokines (regulatory proteins), which
regulate/modify activity of cells.
•Some (interferons), signal cells to produce proteins
that inhibit viral replication (makes the virus weak).
•Other interferons – enhance the abilities of other
immune cells (WBC’s)
•Interleukins (cytokines released from macrophages
and lymphocytes) and histamine (released from
certain WBC’s and injured cells), trigger
inflammatory responses

11. The Inflammatory Response
• Causes blood vessels to dilate:
– Increases blood flow, temperature, redness, the
numbers of phagocytic cells to the invasion site
• Capillaries become more permeable:
– Increases fluid into tissues, edema (swelling) and
pain, antibodies move to tissue
• Widespread inflammatory response = fever
– Thermostat in the hypothalamus is reset via
interleukin produced by macrophages

12. How fever helps
• Interferes with microbe growth
• Promotes the activity of lymphocytes (T cells)
and the production of antibodies
• May increase phagocytosis
A short term low fever may therefore help
speed recovery.

13. Phagocytes
White blood cells that “eat” foreign cells
•Neutrophils and macrophages
•Inflammatory response increases activity of
these cells to “eat” invaders by:
– Increasing blood flow to infected areas
– Increased blood carries more WBC’s to area
– Causes localised swelling and pain with response