12.5 (4.1.1) Non-specific animal defences against pathogens

immune system

a group of cells, tissues, organs and mechanisms that defend an organism against pathogens and other foreign substances

immune response

a complex series of specific and nonspecific processes involving a range of cells and chemicals

skin as a physical barrier

prevents entry of bacteria

produces sebum which inhibits pathogen growth (antiseptic)

skin flora as a physical barrier

large population of natural healthy bacteria that outcompete pathogens for surface space

mucous membranes as a physical barrier

traps pathogens

contains lysosomes that destroy pathogens

steps of blood clotting

1) platelets rush to site releasing thromboplastin and serotonin

2) thromboplastin catalyses the conversion of prothrombin into thrombin (in the presence of Ca²⁺ ions)

3) thrombin catalyses the conversion of fibrinogen to form fibrin

4) fibrin forms a clot

5) clot dries out, forming a tough scab that keeps pathogens out

thromboplastin

enzyme that triggers the conversion of prothrombin into thrombin (in the presence of Ca²⁺ ions) resulting in the formation of a blood clot

serotonin

makes smooth muscle in the walls of blood vessel contract so they narrow and reduce supply of blood to the damaged area to reduce blood loss and promotes platelet formation

blood clotting cascade diagram

inflammation

the swelling of skin

what doe mast cells release

histamines and cytokines

histamines

make blood vessels dilate causing localised heat and redness preventing pathogens reproducing

increase permeability of the blood vessel walls, causing more tissue fluid to escape causing swelling and pain

cytokines

attract white blood cells to the site, disposing of the pathogens by phagocytosis

how do fevers protect against pathogens

cytokines stimulate hypothalamus to increasing the body’s setpoint for temperature

body temp increases

higher temps inhibit pathogen reproduction

specific immune system works faster at a higher temp

steps of phagocytosis

1) pathogens release chemicals that attract phagocytes

2) phagocytes recognise non-human proteins on pathogen

3) phagocyte engulfs pathogen and encloses it a vacuole called a phagosome

4) phagosome combines with a lysosome to form a phagolysosome

5) enzymes from the lysosome digest and destroy the pathogen

6) antigens from the digested pathogen combine with MHC in the cytoplasm

7) MHC/antigen complex is displayed on phagocyte plasma membrane, making an antigen-presenting cell

diagram of steps of phagocytosis

major histocompatibility complex MHC

special glycoproteins in the cytoplasm which combine with antigens from the pathogen during phagocytosis

moves the pathogen antigens to the macrophages own CSM becoming an antigen-presenting cell

macrophage

specialised phagocytes

take longer to break pathogens down than normal phagocytosis

form MHC

how to make a blood smear

spread a drop of blood very thinly across a slide

opsonins

chemicals that bind to pathogens and ‘tag’ them so they can be more easily recognised by pathogens