6.2.1Unit 6: Practical Skills in Biology II
Tuberculosis and HIV — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 7 of 48 Unit quiz
Section 1 of 5Tuberculosis
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TB is an infectious disease caused by the bacterium Mycobacterium tuberculosis.
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An infected person with active TB releases bacteria in droplets when coughing or sneezing, and others inhale them.
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Overcrowded living conditions increase the rate of spread.
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In the lungs the bacteria are engulfed by phagocytes but may survive; in a healthy person they become enclosed in tubercles and stay dormant.
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If the immune system weakens, the bacteria become active, causing fever, fatigue, coughing and lung damage that can be fatal.
Section 2 of 5HIV Transmission
Pathogen
- HIV is the human immunodeficiency virus, a retrovirus with an RNA genome.
Body fluids
- It spreads in body fluids, including through sexual contact, transfused blood and shared needles.
Mother to child
- It can pass across the placenta, by mixing of blood at birth, or through breast milk.
Wording
- Refer to HIV and not the HIV virus, since the V already stands for virus.
Section 3 of 5Replication in Helper T Cells
Replication in Helper T Cells
Entry
- HIV attaches to a receptor on helper T cells and its capsid enters the cell.
Reverse transcription
- Reverse transcriptase makes DNA from the viral RNA, and this DNA is inserted into the host DNA.
Production
- The host cell machinery builds new viral components which assemble into new viruses.
Release
- New viruses bud from the cell and infect other helper T cells.
Section 4 of 5From HIV to AIDS
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Soon after infection there may be flu-like symptoms during the acute stage.
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Replication then slows and there is a long period without symptoms, during which helper T cells gradually fall in number.
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Fewer helper T cells means B cells are not activated and fewer antibodies are made.
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The patient is then prone to opportunistic infections, which are diseases that a healthy immune system would control.
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AIDS is diagnosed when infections become severe or helper T cell numbers fall below a critical level, and it is the opportunistic infections that cause death.
Section 5 of 5Exam Checklist & Tips
6.2.1 – Key Points for the Exam
- Pathogen An organism or virus that causes disease.
- Tubercle A structure in the lung that encloses TB bacteria and keeps them dormant.
- Helper T cell A white blood cell that activates B cells and other cells of the immune response.
- Opportunistic infection An infection that takes hold because the immune system is weakened.
Common Mistakes
- Using HIV and AIDS as if they mean the same; HIV is the virus and AIDS is the condition it can cause.
- Saying HIV kills the patient directly, when death results from opportunistic infections.
- Stating that TB is caused by a virus; the pathogen is a bacterium.
- Forgetting that HIV infects helper T cells, which are central to the immune response.
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