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Given below is the… (Class 12 Biology SQP 2024-25, Q19)

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BiologyCBSE Class 12 · Sample paper, session 2024-25 · 2 marks · Very short answer

CBSE Class 12 Biology sample paper 2024-25: question 19

From the official CBSE sample question paper for 2024-25, section B, question 19, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q19 · 2 marks · Very short answerCompetency-basedHuman Health and Disease

Given below is the relationship between the HIV levels in the blood and helper T cell count in a person detected with AIDS. Study the relationship and answer the questions that follow.

A. What kind of relationship is observed in the virus levels and the immune response after some days of the initial infection?
B. Does it completely clear the virus from the body permanently? Give reason for your answer.

Figure in the paper: Graph of concentration in blood (y-axis) vs time (x-axis, months then years). Virus cultured from blood (blue) rises sharply in first weeks, falls sharply, stays low for a long period, then rises steadily over years. T cell count (red) falls initially, recovers to a plateau, stays steady, then declines steadily over the years, crossing the virus curve. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

Open one hint at a time and try again before the next.

Hint 1

Trace the two curves on the graph during the first few months.

Hint 2

What happens to both curves several years later?

Hint 3

Recall which cells HIV infects and uses to multiply.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. A. A reciprocal (inverse) relationship is seen. As the adaptive immune response builds up, the helper T-cell count recovers and the virus level in blood falls sharply. As helper T levels rise, virus levels decline.

    1 mark

  2. B. No, the virus is not cleared permanently. HIV survives inside macrophages, which act as an 'HIV factory', and inside helper T-lymphocytes. Over the years it keeps replicating and destroys helper T cells, so their number falls progressively while virus levels rise again. If untreated, the person loses the adaptive immune response, including the ability to make antibodies, and develops AIDS.

    1 mark

Answer: A. Reciprocal relationship: as helper T cells rise, virus level falls. B. No; HIV persists in macrophages and T cells and progressively destroys helper T cells, so virus levels rise again.

Where the marks are

  • Describes the reciprocal relationship: rise in helper T cells with fall in virus level (1 mark)
  • States No, with reason: HIV persists and progressively destroys helper T cells, causing loss of immunity (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Human Health and Disease, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.