White Blood Cells & Immunity (Edexcel IGCSE Biology: Double Science)

• White blood cells are part of the body’s immune system
• These specialised cells defend against pathogenic microorganisms
• There are two main types of white blood cell:
  • Phagocytes
  • Lymphocytes

Phagocytes

• Phagocytes carry out phagocytosis by ingesting pathogens
  • Phagocytes have a sensitive cell surface membrane that can detect chemicals produced by pathogenic cells
  • Once they encounter the pathogenic cell, they will engulf it and release digestive enzymes to digest it
  • This is a non-specific immune response

The process of phagocytosis

Lymphocytes

• Lymphocytes produce antibodies
  • Antibodies are proteins with a shape that is specific (complementary) to the antigens on the surface of the pathogen
• Lymphocytes provide a specific immune response as the antibodies produced will only fit one type of antigen on a pathogen

The lymphocytes produce antibodies that are specific to the antigen on the pathogen

• The body's immune system is highly complex, with white blood cells being the main component
• Once a pathogen has entered the body the role of the immune system is to prevent the infectious organism from reproducing and to destroy it
• An organism has immunity when they have sufficient levels of antibodies to protect it from a particular disease
  • As a result, they do not suffer from the disease or its symptoms

Response to infection

• The stages of infection and the subsequent immune response are as follows:
  1. The pathogen enters the blood stream and multiplies
  2. A release of toxins (in the case of bacteria) and infection of body cells causes symptoms in the patient
  3. Phagocytes that encounter the pathogen recognise that it is an invading pathogen and engulf and digest (non-specific response)
  4. Eventually, the pathogen encounters a lymphocyte which recognises its antigens
  5. The lymphocyte starts to produce specific antibodies to combat that particular pathogen
  6. The lymphocyte also clones itself to produce lots of lymphocytes (all producing the specific antibody required)
  7. Antibodies destroy pathogens
  8. Phagocytes engulf and digest the destroyed pathogens