Unit 6 - Human Physiology
6.3 - Defence Against Infectious Disease
6.3.1 - Surface Barriers
The rst line of defence are the surface barriers:
• Skin: protects external structures
• Consists of dead surface cells
• Contains glands that secrete enzymes and chemicals which inhibit microbial growth on skin
• Secretes lactic acid and fatty acids to lower pH
• Mucous membranes: protects internal structures
• Consists of living surface cells that release uids to wash away pathogens (e.g mucuous,
saliva, tears)
• Secrete lysozymes which destroy cell walls
• May possess cilia to help remove pathogens
6.3.2 - Clotting
Clotting: repairs damaged blood vessels to prevent blood loss and block pathogens from
entering blood stream
Key components of blood clots:
• Platelets: become sticky to plug damage
• Fibrin strands: form mesh to trap blood cells
Coagulation cascade: set of reactions involved in blood clotting
1. Clotting factors cause platelets to become sticky and plug damaged area
2. Factors cause vessel to constrict, reducing blood ow
3. Factors activate thrombin, which causes brinogen to transform into brin (insoluble)
4. Fibrin strands form mesh to trap blood cells
5. After reparation, an enzyme dissolves the clot
Coronary thrombosis: formation of blood clot in coronary arteries
If blood clot blocks artery (occlusion), a heart attack may occur.
Caused by atherosclerosis:
• Atheromas (fatty deposits) reduce diameter of lumen
• Increased blood pressure damages arterial wall
• After reparation, elasticity decreases
• Over time, lesions/plaque forms
• If plaque ruptures, blood clotting occurs, creating a thrombus that restricts blood ow
6.3.3 - Phagocytes
The second line of defence is the innate immune system.
• Phagocytes (white blood cells): engulf /digest foreign bodies
• In ammation
• Fever
• Antimicrobial chemicals
Main properties:
fl fi fifl fl fi fl
, • Non-speci c: doesn’t di erentiate between di erent types of pathogens
• Non-adaptive: responds to infection same same way every time
Phagocytosis: process where phagocyte ingests pathogens (via endocytosis)
Di erent types of phagocytes include macrophages and dendritic cells.
• Phagocytes circulate through blood stream
• Damaged body tissue releases chemicals that attract white blood cells
• Phagocyte engulf pathogen, trapping it in an internal vesicle
• Lysosome (organelle) fuses with vesicle and digests pathogen
• Fragments of pathogen may be presented as antigens (stimulating third line of defence)
6.3.4 - Lymphocytes
The third line of defence is the adaptive immune system.
Main properties:
• Speci c: can di erentiate between and target speci c pathogens
• Adaptive: responds faster when re-exposed to a pathogen (building immunity)
Lymphocytes: cells that aid in the production of antibodies
• B cells: recognise and target speci c antigens to produce antibodies
• T cells: regulatory, release cytokines to activate B cells
Process:
• Some phagocytes (dendritic cells) present antigens on surface
• Dendritic cells migrate to lymph nodes and activate T cells
• T cells release cytokines that activate B cells
• B cells di erentiate into plasma cells that produce speci c antibodies
• Antibodies bind to speci c antigens and cause them to agglutinate (stick together), making
them easier to recognise and be destroyed
• Small portion of lymphocytes develop into memory cells to provide long-lasting immunity
Antigen: pathogen fragments that elicit an immune response
Antibody: protein speci c to an antigen
Antigens form Y-shaped molecules with unique antigen-binding sites at the ends.
ff
fi ff fi ff fififf fi ff fi fi
6.3 - Defence Against Infectious Disease
6.3.1 - Surface Barriers
The rst line of defence are the surface barriers:
• Skin: protects external structures
• Consists of dead surface cells
• Contains glands that secrete enzymes and chemicals which inhibit microbial growth on skin
• Secretes lactic acid and fatty acids to lower pH
• Mucous membranes: protects internal structures
• Consists of living surface cells that release uids to wash away pathogens (e.g mucuous,
saliva, tears)
• Secrete lysozymes which destroy cell walls
• May possess cilia to help remove pathogens
6.3.2 - Clotting
Clotting: repairs damaged blood vessels to prevent blood loss and block pathogens from
entering blood stream
Key components of blood clots:
• Platelets: become sticky to plug damage
• Fibrin strands: form mesh to trap blood cells
Coagulation cascade: set of reactions involved in blood clotting
1. Clotting factors cause platelets to become sticky and plug damaged area
2. Factors cause vessel to constrict, reducing blood ow
3. Factors activate thrombin, which causes brinogen to transform into brin (insoluble)
4. Fibrin strands form mesh to trap blood cells
5. After reparation, an enzyme dissolves the clot
Coronary thrombosis: formation of blood clot in coronary arteries
If blood clot blocks artery (occlusion), a heart attack may occur.
Caused by atherosclerosis:
• Atheromas (fatty deposits) reduce diameter of lumen
• Increased blood pressure damages arterial wall
• After reparation, elasticity decreases
• Over time, lesions/plaque forms
• If plaque ruptures, blood clotting occurs, creating a thrombus that restricts blood ow
6.3.3 - Phagocytes
The second line of defence is the innate immune system.
• Phagocytes (white blood cells): engulf /digest foreign bodies
• In ammation
• Fever
• Antimicrobial chemicals
Main properties:
fl fi fifl fl fi fl
, • Non-speci c: doesn’t di erentiate between di erent types of pathogens
• Non-adaptive: responds to infection same same way every time
Phagocytosis: process where phagocyte ingests pathogens (via endocytosis)
Di erent types of phagocytes include macrophages and dendritic cells.
• Phagocytes circulate through blood stream
• Damaged body tissue releases chemicals that attract white blood cells
• Phagocyte engulf pathogen, trapping it in an internal vesicle
• Lysosome (organelle) fuses with vesicle and digests pathogen
• Fragments of pathogen may be presented as antigens (stimulating third line of defence)
6.3.4 - Lymphocytes
The third line of defence is the adaptive immune system.
Main properties:
• Speci c: can di erentiate between and target speci c pathogens
• Adaptive: responds faster when re-exposed to a pathogen (building immunity)
Lymphocytes: cells that aid in the production of antibodies
• B cells: recognise and target speci c antigens to produce antibodies
• T cells: regulatory, release cytokines to activate B cells
Process:
• Some phagocytes (dendritic cells) present antigens on surface
• Dendritic cells migrate to lymph nodes and activate T cells
• T cells release cytokines that activate B cells
• B cells di erentiate into plasma cells that produce speci c antibodies
• Antibodies bind to speci c antigens and cause them to agglutinate (stick together), making
them easier to recognise and be destroyed
• Small portion of lymphocytes develop into memory cells to provide long-lasting immunity
Antigen: pathogen fragments that elicit an immune response
Antibody: protein speci c to an antigen
Antigens form Y-shaped molecules with unique antigen-binding sites at the ends.
ff
fi ff fi ff fififf fi ff fi fi
