Lesson 4 of 7 / Transport in humans
Immune defence and blood clotting
How do white cells and platelets protect the body in different ways?
I can explain phagocytosis, antibodies, tissue rejection and blood clotting.
I can state blood components and all listed transport and defence roles.Syllabus
Syllabus K327 / K328, Topic 5(c). State blood components and all listed transport and defence roles.
About 5 min
Make a guess. You are not marked.
Do all white blood cells fight pathogens by making antibodies?
Show the answer
No. Some engulf pathogens; others make antibodies.
Phagocytes engulf and digest pathogens. Lymphocytes are the white cells that can make antibodies.
The key idea
White cells recognise and respond to foreign material; platelets help form a fibrin clot at damaged vessels.
Simplified models of different responses; symbols and times are not to scale.
Explanation
Some white blood cells act as phagocytes. They surround and engulf a pathogen, enclosing it before enzymes digest it. This removes the pathogen rather than carrying it around the body.
Other white blood cells, lymphocytes, can make antibodies. An antibody binds to a matching antigen on a pathogen or foreign material. This can neutralise the target or help other defence processes remove it. Antibodies are specific, so one type does not bind every possible antigen.
The same recognition of foreign antigens can cause tissue rejection. Cells in a transplanted organ may have antigens different from those of the recipient. Immune cells can recognise the transplanted tissue as foreign and damage it. Rejection is not the body failing to recognise all cells.
Platelets are small cell fragments. At a damaged vessel they help form a plug and trigger clotting reactions. These reactions convert soluble fibrinogen in plasma into insoluble fibrin threads. The mesh traps blood cells, helping form a clot.
A clot reduces blood loss and helps block pathogen entry through a wound. It is not made by red blood cells turning into fibrin. Detailed names of the clotting enzymes are not needed for this outcome.
Step by step
- 1
Identify the problem
A pathogen, foreign tissue and a damaged vessel require different explanations.
- 2
Name the component
Choose a phagocyte, an antibody-producing lymphocyte or platelets.
- 3
Describe the action
Use engulf and digest, recognise and bind, or trigger formation of a fibrin mesh.
Worked example
Closing a small wound
Explain how platelets and a plasma protein reduce blood loss from a damaged vessel.
One way to explain it
Platelets accumulate at the damaged region and help trigger clotting. Soluble fibrinogen is converted into insoluble fibrin. Fibrin threads form a mesh that traps blood cells, producing a clot that reduces further blood loss.
Why this answer works
- Platelets start and support the response.
- Distinguish soluble fibrinogen from insoluble fibrin.
- Explain how the mesh helps block the damaged site.
Watch out for this
A student says: "All white blood cells protect the body by producing antibodies." What is wrong with this?
Show the answer
Different white cells have different roles. Phagocytes engulf and digest; some lymphocytes produce antibodies.
Check your understanding
Why can the immune system damage a transplanted organ?
Show the answer
Its cells may carry antigens recognised as foreign by the recipient.
Immune recognition of different antigens can lead to tissue rejection.
Quick recall
How do white cells and platelets protect the body in different ways?