K224 / K225 / 2027

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 K224 / K225, 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?

  1. Yes. Making antibodies is what white cells do.
  2. No. Some engulf pathogens; others make antibodies.
  3. No. White cells make clots that trap pathogens.
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.

phagocytosis: before the responseBacteriumA phagocyte approaches
Predict which blood component responds, then reveal what it does.

Simplified models of different responses; symbols and times are not to scale.

Explanation

Some white blood cells are phagocytes. A phagocyte surrounds a pathogen and takes it in. Then enzymes inside the phagocyte digest it.

Other white blood cells, called lymphocytes, make antibodies. An antibody fits onto a matching antigen on a pathogen. This can stop the pathogen working, or help other cells destroy it. Each antibody fits only one kind of antigen.

This same recognition can cause problems after a transplant. Cells in a new organ may have different antigens from the person's own cells. White blood cells may recognise the organ as foreign and attack it. This is rejection.

Platelets are small pieces of cells. At a cut blood vessel, they help form a plug and start clotting. Clotting turns soluble fibrinogen in the plasma into insoluble fibrin threads. The threads form a mesh that traps blood cells and makes a clot.

A clot reduces blood loss and helps stop pathogens getting in through the wound.

Step by step
  1. 1

    Name the problem

    A pathogen, a transplanted organ and a cut blood vessel each need a different answer.

  2. 2

    Name the part of the blood

    Choose a phagocyte, a lymphocyte that makes antibodies, or platelets.

  3. 3

    Say what it does

    Phagocytes engulf and digest. Antibodies recognise and bind. Platelets help form a mesh of fibrin.

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?

  1. Its cells may carry antigens recognised as foreign by the recipient.
  2. Its red cells convert directly into fibrin.
  3. The recipient's red blood cells clot inside the new organ and block its blood supply.
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

Card 1 of 3. Answer in your head, then check.

How do white cells and platelets protect the body in different ways?

You can return to this lesson any time.