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Q11:
What is the mechanism by which the AIDS virus causes deficiency of immune system of the infected person?
Solution :
1. Viral Entry and Initial Infection
The Human Immunodeficiency Virus (HIV), a member of the group Retroviridae, initiates infection by targeting cells expressing the $CD4$ receptor on their surface, primarily Helper T-lymphocytes ($T_H$ cells) and macrophages. The viral envelope glycoprotein $gp120$ binds with high affinity to the $CD4$ receptor and co-receptors (such as $CCR5$ or $CXCR4$), facilitating the fusion of the viral envelope with the host plasma membrane [Per the mechanisms of viral entry in enveloped RNA viruses].
2. Reverse Transcription and Integration
Upon entry into the cytoplasm, the viral core releases its single-stranded RNA (ssRNA) genome and enzymes. Using the enzyme reverse transcriptase, the viral RNA is transcribed into complementary DNA (cDNA). This cDNA then enters the host nucleus and is integrated into the host genome via the enzyme integrase, forming a provirus [Per the Central Dogma of Molecular Biology and Retroviral Replication Cycle].
3. Viral Replication and Cytopathic Effect
Once integrated, the host cell’s machinery is hijacked to transcribe and translate viral genes, producing new viral components. These components assemble at the plasma membrane, and new virions are released via budding. This process eventually leads to the death of the host $T_H$ cell, either through direct viral-mediated lysis or through programmed cell death (apoptosis) triggered by the accumulation of viral proteins [Per the Cytopathic Effect hypothesis].
4. Quantitative Depletion of Helper T-Lymphocytes
The continuous production of virions leads to a systemic reduction in the population of $CD4^+$ Helper T-cells. Because $T_H$ cells are the "command centers" of the immune system—coordinating both the Cell-Mediated Immunity (via Cytotoxic T-cells) and the Humoral Immunity (via B-cell antibody production)—their depletion cripples the entire adaptive immune response.
5. Clinical Consequence: Immunodeficiency
As the $CD4$ count drops below the critical threshold (typically $< 200 \text{ cells/mm}^3$), the infected person becomes highly susceptible to opportunistic infections by pathogens such as Mycobacterium, fungi, viruses, and parasites like Toxoplasma, as well as rare cancers like Kaposi's sarcoma. This state of severe immune dysfunction is defined as Acquired Immuno-Deficiency Syndrome (AIDS).
| Process | Mechanism |
|---|---|
| Viral Entry | Binding to CD4 receptors on T-Helper cells. |
| Genetic Integration | Reverse transcription of RNA to DNA; integration into host genome. |
| Immune Suppression | Progressive decline in T-Helper cell count, crippling signaling for B-cells and Cytotoxic T-cells. |
Final Solution: The HIV virus causes immunodeficiency by specifically infecting and destroying $CD4^+$ Helper T-lymphocytes. Through the processes of viral replication, integration, and host-cell lysis, the population of these regulatory immune cells is drastically reduced. This prevents the activation of both Humoral and Cell-Mediated immunity, leaving the host vulnerable to lethal opportunistic infections.
More Questions from Class 12 Biology Human Health and Disease EXERCISES
- Q1: What are the various public health measures, which you would suggest as safeguard against infectious diseases?
- Q1: What are the various public health measures, which you would suggest as safeguard against infectious diseases?
- Q2: In which way has the study of biology helped us to control infectious diseases?
- Q2: In which way has the study of biology helped us to control infectious diseases?
- Q3(a): How does the transmission of each of the following diseases take place? (a) Amoebiasis
- Q3(a): How does the transmission of each of the following diseases take place? (a) Amoebiasis
- Q3(b): How does the transmission of each of the following diseases take place? (b) Malaria
- Q3(b): How does the transmission of each of the following diseases take place? (b) Malaria
- Q3(c): How does the transmission of each of the following diseases take place? (c) Ascariasis
- Q3(c): How does the transmission of each of the following diseases take place? (c) Ascariasis
- Q3(d): How does the transmission of each of the following diseases take place? (d) Pneumonia
- Q3(d): How does the transmission of each of the following diseases take place? (d) Pneumonia
- Q4: What measure would you take to prevent water-borne diseases?
- Q4: What measure would you take to prevent water-borne diseases?
- Q5: Discuss with your teacher what does ‘a suitable gene’ means, in the context of DNA vaccines.
- Q5: Discuss with your teacher what does ‘a suitable gene’ means, in the context of DNA vaccines.
- Q6: Name the primary and secondary lymphoid organs.
- Q6: Name the primary and secondary lymphoid organs.
- Q7(a): The following are some well-known abbreviations, which have been used in this chapter. Expand each one to its full form: (a) MALT
- Q7(a): The following are some well-known abbreviations, which have been used in this chapter. Expand each one to its full form: (a) MALT
- Q7(b): The following are some well-known abbreviations, which have been used in this chapter. Expand each one to its full form: (b) CMI
- Q7(b): The following are some well-known abbreviations, which have been used in this chapter. Expand each one to its full form: (b) CMI
- Q7(c): The following are some well-known abbreviations, which have been used in this chapter. Expand each one to its full form: (c) AIDS
- Q7(c): The following are some well-known abbreviations, which have been used in this chapter. Expand each one to its full form: (c) AIDS
- Q7(d): The following are some well-known abbreviations, which have been used in this chapter. Expand each one to its full form: (d) NACO
- Q7(d): The following are some well-known abbreviations, which have been used in this chapter. Expand each one to its full form: (d) NACO
- Q7(e): The following are some well-known abbreviations, which have been used in this chapter. Expand each one to its full form: (e) HIV
- Q7(e): The following are some well-known abbreviations, which have been used in this chapter. Expand each one to its full form: (e) HIV
- Q8(a): Differentiate the following and give examples of each: (a) Innate and acquired immunity
- Q8(a): Differentiate the following and give examples of each: (a) Innate and acquired immunity
- Q8(b): Differentiate the following and give examples of each: (b) Active and passive immunity
- Q8(b): Differentiate the following and give examples of each: (b) Active and passive immunity
- Q9: Draw a well-labelled diagram of an antibody molecule.
- Q9: Draw a well-labelled diagram of an antibody molecule.
- Q10: What are the various routes by which transmission of human immuno-deficiency virus takes place?
- Q10: What are the various routes by which transmission of human immuno-deficiency virus takes place?
- Q11: What is the mechanism by which the AIDS virus causes deficiency of immune system of the infected person?
- Q12: How is a cancerous cell different from a normal cell?
- Q12: How is a cancerous cell different from a normal cell?
- Q13: Explain what is meant by metastasis.
- Q13: Explain what is meant by metastasis.
- Q14: List the harmful effects caused by alcohol/drug abuse.
- Q14: List the harmful effects caused by alcohol/drug abuse.
- Q15: Do you think that friends can influence one to take alcohol/drugs? If yes, how may one protect himself/herself from such an influence?
- Q15: Do you think that friends can influence one to take alcohol/drugs? If yes, how may one protect himself/herself from such an influence?
- Q16: Why is that once a person starts taking alcohol or drugs, it is difficult to get rid of this habit? Discuss it with your teacher.
- Q16: Why is that once a person starts taking alcohol or drugs, it is difficult to get rid of this habit? Discuss it with your teacher.
- Q17: In your view what motivates youngsters to take to alcohol or drugs and how can this be avoided?
- Q17: In your view what motivates youngsters to take to alcohol or drugs and how can this be avoided?
CBSE Solutions for Class 12 Biology Human Health and Disease
Chapters in CBSE - Class 12 Biology
- Reproduction in Organisms
- Human Reproduction
- Reproductive Health
- Principles of Inheritance and Variation
- Molecular Basis of Inheritance
- Evolution
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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