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2.6.3Unit 2 — Multicellular Organisms

White Blood Cells

5 min read 5 parts Lesson 29 of 39

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Part 1 of 5White Blood Cells and Immunity

2.6.3 White Blood Cells
White Blood Cells and Immunity
Immune system
  • White blood cells are part of the immune system and help to destroy pathogens that have entered the body.
Pathogens
  • Pathogens include bacteria, viruses and fungi, and the immune system must stop them reproducing and destroy them.
Amount
  • White blood cells make up less than one per cent of the total volume of the blood.
Two main types
  • The two main types are phagocytes, which engulf pathogens, and lymphocytes, which make antibodies.

Part 2 of 5Phagocytes and Phagocytosis

1
A phagocyte has a sensitive cell surface membrane that can detect chemicals produced by a pathogen.
2
The phagocyte moves towards the pathogen and surrounds it by engulfing it.
3
The pathogen is now enclosed inside the phagocyte, where digestive enzymes are released onto it.
4
The enzymes digest and destroy the pathogen, which is a non-specific response because it works on many types of pathogen.

Part 3 of 5Lymphocytes and Antibodies

Antigen
  • A marker on the surface of a cell or pathogen that the body recognises as foreign and responds to.
Antibody
  • A protein produced by lymphocytes with a shape complementary to one particular antigen.
Specific response
  • Each antibody fits only one type of antigen, so the response is specific to one type of pathogen.
Result
  • Antibodies bind to the pathogens and help to destroy them or mark them out for destruction.
Cell typeMethod of defenceType of response
PhagocyteEngulfs and digests pathogensNon-specific
LymphocyteProduces antibodiesSpecific

Part 4 of 5Defence in Action

Defence in Action
First contact
  • When a pathogen enters the body, phagocytes can act at once because they do not need to recognise a specific antigen.
Targeted help
  • Lymphocytes take longer to respond, but their antibodies target only the pathogen that carries the matching antigen.
Preventing spread
  • Stopping pathogens from reproducing quickly limits the damage that they can do to the body’s cells.
Working together
  • The two cell types complement one another, since phagocytes give a general defence and lymphocytes a precise one.

Part 5 of 5Exam Checklist & Tips

2.6.3 – Key Points for the Exam

  • Pathogen A microorganism that causes disease.
  • Phagocyte A white blood cell that engulfs and digests pathogens.
  • Lymphocyte A white blood cell that produces antibodies.
  • Antigen A marker on a cell surface that is recognised as foreign.
  • Antibody A protein that is complementary to a specific antigen.

Common Mistakes

  • Mixing up antigen and antibody; antigens are on pathogens and antibodies are made by lymphocytes.
  • Saying that phagocytes produce antibodies, which is the role of lymphocytes.
  • Describing phagocytosis as a specific response, when it is non-specific.
  • Stating that an antibody can fit any antigen, rather than only the complementary one.
Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

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