Immunity (Edexcel GCSE Biology: Combined 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 - carry out phagocytosis
  • Lymphocytes - produce antibodies and antitoxins

Phagocytes

• Phagocytes carry out phagocytosis by engulfing and digesting 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 as the response is the same for any pathogenic cell

• Phagocytes can be easily recognised under the microscope by their multi-lobed nucleus and their granular cytoplasm

The process of phagocytosis

Lymphocytes

• B-Lymphocytes can easily be recognised under the microscope by their large round nucleus which takes up nearly the whole cell and their clear, non-granular cytoplasm
• B-Lymphocytes produce antibodies
  • Antibodies are Y-shaped proteins with a shape that is specific (complementary) to the antigens on the surface of the pathogen
  • This is a specific type of 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

• Antibodies attach to the antigens and cause agglutination (clumping together)
• This means the pathogenic cells cannot move very easily
• At the same time, chemicals are released that signal to phagocytes that there are cells present that need to be destroyed

Agglutinated pathogens cannot move easily

• Lymphocytes also produce antitoxins to neutralise toxins released by pathogens

Lymphocytes produce specific antitoxins to specific pathogens

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 B-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 cause agglutination of pathogens
  8. Phagocytes engulf and digest the agglutinated pathogens
  9. After the patient has recovered, they retain antibodies specific to the disease as well as memory cells (lymphocytes that recognise the pathogen)
  10. If the patient encounters the same pathogen again, it will trigger a secondary immune response
  11. Memory cells can produce much larger quantities of the required antibody in a much shorter time to fight off the pathogen before the patient suffers any symptoms