Edexcel A Level (IAL) Biology -6.6 Tuberculosis & HIV- Study Notes- New Syllabus

How Mycobacterium tuberculosis and HIV Infect Human Cells and Cause Disease

🌱 Introduction

Two major pathogens cause serious human diseases:

• Mycobacterium tuberculosis (bacterium) causes TB.
• HIV (virus) causes AIDS.
• Both target specific cells and weaken essential body systems, leading to symptoms that may become fatal.

🦠 Mycobacterium tuberculosis (TB)

How TB Infects the Body

• Enters the body mainly by inhalation of droplets from an infected person.
• Bacteria reach the alveoli in the lungs.
• Macrophages attempt to engulf them by phagocytosis.
• TB bacteria survive inside macrophages, avoiding destruction.
• They multiply slowly inside these cells.

Formation of Granulomas (Tubercules)

• The immune system builds a wall of cells around infected macrophages.
• This forms granulomas, areas of infected but contained tissue.
• TB can stay dormant for years.

🩺 Symptoms of Active TB

When the immune system weakens, bacteria break out of granulomas and spread.

• Persistent cough
• Coughing blood
• Chest pain
• Fever and night sweats
• Weight loss
• Fatigue
• Shortness of breath

⚠️ How TB Causes Death

• Severe lung damage reduces gas exchange.
• Spread to other organs like kidneys, bones, brain (miliary TB).
• Respiratory failure or organ failure can occur.
• Vulnerable groups (elderly, weak immunity) are at highest risk.

🧬 HIV (Human Immunodeficiency Virus)

How HIV Infects the Body

• Spread through blood, semen, vaginal fluids, shared needles, or from mother to child.
• Targets CD4 helper T cells, a key part of the immune system.

Infection Steps

• Attachment
  HIV glycoprotein (gp120) binds to CD4 receptors on T helper cells.
• Fusion & Entry
  Virus envelope fuses with the cell membrane.
• Reverse Transcription
  Viral RNA is converted into DNA by reverse transcriptase.
• Integration
  Viral DNA enters host genome using integrase.
• Virus may stay latent for years.
• Replication & Release
  Activation leads to new virus particles being produced. CD4 cells eventually die, reducing immune function.

🩺 Symptoms of HIV/AIDS

Early infection:

• Mild flu-like symptoms
• Swollen lymph nodes

Latency:

• Often no symptoms for several years

AIDS (late stage):

• Extremely low CD4 count
• Recurrent infections (TB, pneumonia, fungal infections)
• Weight loss
• Chronic diarrhoea
• Cancers like Kaposi’s sarcoma

⚠️ How HIV Causes Death

• The immune system becomes severely weakened.
• The person cannot fight off even minor infections.
• Opportunistic infections or cancers become fatal.
• Respiratory failure, systemic infections, or organ damage can lead to death.

📋 Summary Table