Cells And The Immune System

Question 1

What are antigens?

Answer 1

• Molecules (usually proteins) that can generate an immune response.
• Found on the surface of cells (including all your body cells).

Question 2

What is a foreign antigen?

Answer 2

An antigen that isn’t normally found in the body, the foreign antigens are what the immune system responds to.

Question 3

State and explain what antigens allow the immune system to identify:

Answer 3

1. Pathogens:
   • organisms that cause disease e.g. bacteria.
   • All pathogens contain antigens on their surface, which are recognised as foreign by the immune system cells, generating an immune response.
2. Abnormal body cells:
   • Cancerous or pathogen-infected cells have abnormal antigens on their surface, triggering an immune response.
3. Toxins (poisons):
   • Molecules, not cells.
4. Cells from other individuals of the same species:
   • When you receive cells from another person e.g. through a blood transfusion, these cells contain antigens different to your own, triggering an immune response.
   • This response leads to the rejection of transplanted organs if drugs are not taken to suppress the recipients immune system.

Question 4

What are the most important antigens for a blood transfusion?

Answer 4

ABO antigens; if the donated blood contains A or B antigens that aren’t recognised by the recipients immune system, this will trigger an immune response.

Question 5

State and describe the first stage of the immune response:

Answer 5

• Phagocytosis.
  • Phagocyte recognises foreign antigens on pathogen’s surface.
  • Cytoplasm of phagocyte moves around pathogen, engulfing it.
  • Pathogen is now contained in a phagocytic vacuole in the cytoplasm of the phagocyte.
  • Lysosome fuses with phagocytic vacuole; lysozymes break down pathogen.
  • Phagocyte presents pathogen’s antigen’s on its surface; it sticks antigens to its surface - acting as an antigen-presenting cell.

Question 6

State and describe the second stage of the immune response.

Answer 6

• T-Cells
  • These are a type of white blood cell containing receptor proteins on their surface which bind to complementary antigens presented to it by the phagocyte; this activates the T-Cell.
  • Helper T-Cells (TH cells) release chemical signals that activate and stimulate phagocytes.
  • Cytotoxic T-Cells (Tc cells) kill abnormal/foreign cells.
  • Helper T-also activate B-Cells, which release antibodies.

Question 7

State and describe the third stage of the immune response:

Answer 7

• B-Cells:
  • These are a type of white blood cell covered in antibodies on their surface (proteins that bind to complementary antigens to form an antigen-antibody complex).
  • B-Cells are activated by T-Helper cells releasing substances, and antibodies on the surface of B-Cells binding to complementary antigens to form an antigen-antibody complex (this process is called clonal selection.
  • The activated B-Cells then divide into plasma cells.