AQA A Level Biology: Cell Recognition and the Immune System

Overview

• Topic: Cell recognition and immune system
• Part of: Cells Unit
• Includes: Exam-style questions and mark schemes
• Format: Includes timestamps for easy navigation

Cell Recognition

• Self vs. Foreign Cells:
  • Self cells: Cells from your own body
  • Foreign cells: Pathogens, abnormal body cells, toxins, cells from others
• Molecules on Surface:
  • Identification via specific molecules (often proteins)
• Antigens:
  • Definition: Foreign proteins on cell surface that stimulate immune response
  • Purpose: Identifies pathogens

Antigenic Variability

• Pathogens mutate their antigens, changing their structure
• Implications: Immune system may not detect new antigens
• Example: Flu vaccinations need updates due to antigenic variability

Types of Immune Responses

1. Phagocytosis

   • Engulfment of a pathogen by a phagocyte
   • Steps: Attraction, binding, engulfment, formation of phagosome, lysosome fusion, breakdown of pathogen
   • Result: Phagocyte becomes an antigen-presenting cell

2. Cell-mediated Response

   • Involves T cells (mature in thymus)
   • Steps: T cell receptors bind to antigens, T cells differentiate into helper T cells, cytotoxic T cells, or stimulate phagocytosis

3. Humoral Response

   • Involves B cells (mature in bone marrow)
   • Steps: Helper T cells stimulate B cells, clonal selection occurs, plasma cells produce antibodies, destruction of pathogens via agglutination

Antibodies

• Structure:
  • Y-shaped with 4 polypeptide chains (2 heavy, 2 light)
  • Constant and variable regions
• Function:
  • Bind to antigens and destroy pathogens
• Agglutination:
  • Antibody-antigen complex sticks together, easier for phagocytosis

Immune Response

• Primary vs. Secondary Response:
  • Primary: First contact with pathogen
  • Secondary: Memory B cells rapidly produce antibodies on re-infection

Vaccination

• Purpose: Initiates primary response, forming memory B cells
• Herd Immunity: Immunizes majority to protect unvaccinated individuals
• Ethical Issues: Compulsory vaccinations, testing demographics, effectiveness

Immunity Types

• Active Immunity: Body produces antibodies
• Passive Immunity: Antibodies introduced externally (e.g., antivenom, breastfeeding)
  • Artificial vs. Natural Passive Immunity

Monoclonal Antibodies

• Definition: Isolated from single clone of B cells
• Uses:
  • Drug targeting
  • Detection of diseases (e.g., ELISA test)
• Ethical Considerations: Use of mice in production

HIV Structure and Replication

• Structure:
  • Glycoproteins, capsid, RNA, reverse transcriptase
• Replication:
  • Attachment to CD4 receptors, conversion of RNA to cDNA, use of host machinery

HIV and AIDS

• Impact: Kills T cells, reduces immune response, increased vulnerability
• Treatment: Target reverse transcriptase enzyme
• Antibiotics: Ineffective against viruses

Exam-style Questions

• Key Insight: Focus on how pathogens cause disease, vaccination coverage, herd immunity, and specifics of antibody actions.

Common Errors

• Avoiding false negatives in tests
• Understanding specific responses and terms (e.g., resistances, herd immunity)

Conclusion

• Contact: Open for questions regarding A-level biology

This summary provides an overview of the key concepts in the lecture, including the mechanisms of the immune system, the nature and function of antibodies, and practical implementations such as vaccinations and tests.