4.1.4 Non-specific Immune Responses Flashcards
What categories can the different methods of defence be divided into?
Vertebrate animals have developed complex mechanisms to defend themselves against the invasion of pathogens
The different methods of defence can be divided into four categories:
Physical: body tissues act as barriers, preventing the entry of pathogens
E.g. skin, mucous membrane of the alimentary canal
Cellular: cells detect and signal the presence of pathogens. Protective substances are secreted and the pathogens are ingested and digested
Chemical: secreted substances generate an inhospitable environment for the growth of pathogens. These substances can trap pathogens, cause them to burst, or prevent them from entering cells and reproducing
Commensal organisms: the harmless bacteria and fungi present on and in the body compete with pathogens for nutrients.
What is the first line of defence?
A human has three lines of defence
The first line of defence prevents the entry of pathogens and is comprised of the following:
Skin
Mucous membranes
Expulsive reflexes
Chemical secretions
Describe the role of the skin in the first line of defense.
Skin posses an outer layer of dry, dead, hardened cells filled with keratin
Keratin is a tough fibrous protein
This layer of cells acts as a physical barrier to pathogens
There are secretions of sebum that contain fatty acids which have antimicrobial properties
Evaporation of sweat from the skin leaves behind a salt residue
The lack of moisture, low pH and high salinity creates an inhospitable environment for the growth of microorganisms
What is the role of mucous membranes in the first line of defence?
Mucous membranes line the gut, airways and reproductive system
The mucous membrane consists of epithelial cells and mucus-secreting cells like goblet cells
Mucus contains lots of glycoproteins with long carbohydrate chains.
These chains are what make mucus sticky
Viruses, bacteria, pollen and dust float about in the air that we breathe in
Mucus in the airways (trachea, bronchi and bronchioles) can trap these particles
The particles are then moved towards the back of the throat by cilia
Cilia are small hair-like structures on the surface of cells. Some ciliated epithelial cells have motile cilia that beat and move in a wave-like manner to move mucus along the airway
What is the role of explosive reflexes in the first line of defence?
When a pathogen irritates the lining of an airway it can trigger an expulsive reflex; a cough or sneeze
Both a cough and sneeze result in a sudden expulsion of air. This expelled air contains secretions from the respiratory tract along with the foreign particles that have entered
What is the role of chemical secretions in the first line of defense?
Lysozymes are antimicrobial enzymes that breakdown the cell wall of bacteria
These special enzymes are found in body fluids such as blood, tears, sweat, and breast milk
Hydrochloric acid is produced by the cells that line the stomach
The acid creates a low pH inside the stomach which helps to kill any bacteria that has been ingested alongside food
The cells of the gut secrete mucus to prevent being damaged by hydrochloric acid
What is the role of commensal microorganisms in the first line of defence?
On average roughly 1kg of a human’s weight is made up of the bacteria on or inside their body
Candida albicans and E. coli are examples of bacteria commonly found on and in humans
These microorganisms grow on the skin, in the mouth and intestines however they do not cause disease
Their growth is limited by the defence mechanisms
Hosting these microorganisms can have a major benefit for humans
They compete with pathogenic microorganisms and prevent them from invading host tissue
Antibiotics often kill friendly gut bacteria which can allow for opportunistic pathogens to grow
What is the second line of defence?
When a pathogen manages to evade the first line of defence then the second line of defence will respond
The second line of defence involves phagocytic cells and antimicrobial proteins responding to the invading pathogens
Second-line responses include:
Blood clotting
Inflammation
Wound repair
Phagocytosis
What is the role of blood clotting in the second line of defence?
When the body is wounded it responds rapidly
A break in the mucous membranes or skin membranes causes the release of molecules that trigger a chemical cascade which results in blood clotting
Platelets release substances that undergo a series of chemical reactions
The end product is that fibrin is formed, which forms a network, trapping platelets and forming a clot
Blood clotting prevents excess blood loss, the entry of pathogens and provides a barrier (scab) for wound healing to occur
Look at figure in SME
What is the role of inflammation in the second line of defence?
The surrounding area of a wound can sometimes become swollen, warm and painful to touch; this is described as inflammation
Inflammation is a local response to infection and tissue damage. It occurs via chemical signalling molecules which cause the migration of phagocytes into the tissue and increased blood flow
Body cells called mast cells respond to tissue damage by secreting the cell signalling molecule, histamine
Histamine stimulates the following responses:
Vasodilation increases blood flow through capillaries
“Leaky” capillaries allow fluid to enter the tissues and creating swelling
A portion of the plasma proteins leave the blood
Phagocytes leave the blood and enter the tissue to engulf foreign particles
Cells release cytokines that trigger an immune response in the infected area
Cytokines are cell-signalling compounds that stimulate inflammation and an immune response
They are small proteins molecules
Interleukins are a group of cytokines
Interleukin 1 (IL-1) and interleukin 6 (IL-6) promote inflammation
IL-1 targets the brain, causing drowsiness and fever
What is the role of wound repair in the second line of defence?
A scab is formed as a result of blood clotting
Underneath this scab, there are stem cells that divide by mitosis to heal the wound
Wound healing occurs in a number of overlapping stages:
New blood vessels form
Collagen is produced
Granulation tissue forms to fill the wound
Stem cells move over the new tissue and divide to produce epithelial cells
Contractile cells cause wound contraction
Unwanted cells die
What are all the responses above examples of?
All of the above are examples of non-specific immune responses
Non-specific defences are present in humans from birth. The rapid response is the same for every pathogen; they do not distinguish between pathogens
They are not always effective
