6.3.5 The Human Immune System

Non-specific immunity

• The non-specific immune response is comprised of general protective responses carried out by the body
• Phagocytes are involved in the non-specific immune response
  
  • It is a non-specific immune response as the response is the same for any pathogenic cell

Specific immunity

• Lymphocytes are white blood cells that are involved in the specific immune response
• A certain type of lymphocyte called B-Lymphocytes (or sometimes B-cells) 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 immune response as the antibodies produced will only fit one type of antigen on a pathogen
  
  • Therefore, the antibody will only be able to act against a specific antigen

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
• Lymphocytes also produce antitoxins to neutralise toxins released by pathogens

Agglutinated pathogens cannot move easily

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 bloodstream 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 set of antigens
6. The lymphocyte also clones itself to produce millions of copies of itself, 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, which is much faster than the primary 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 displays any symptoms