Human Health and Disease Solutions for Class 12 Biology (2025-26)

To score full marks on this important question, your answer must be structured and complete. Follow these steps:

• Introduction: Start by naming the causative agent (e.g., Plasmodium vivax) and the disease (Malaria). Mention its digenetic nature, requiring two hosts: humans (primary) and female Anopheles mosquito (secondary).

• Life Cycle in Human: Describe the asexual phase. Detail the entry of sporozoites into the bloodstream, their multiplication in liver cells, and subsequent attack on Red Blood Cells (RBCs). Mention the rupture of RBCs and the release of the toxic substance haemozoin, which causes chills and high fever.

• Life Cycle in Mosquito: Describe the sexual phase. Explain the formation of gametocytes in human blood, and their transfer to the mosquito. Detail the fertilization and development within the mosquito's gut to form sporozoites, which migrate to the salivary glands.

• Diagram: A neat, well-labelled diagram showing both hosts and all stages is non-negotiable and carries significant weightage.

This is a classic 3-mark question. To answer effectively, present the differences in a tabular format or with clear headings:

• Source of Antibodies: In active immunity, the body produces its own antibodies in response to an antigen (e.g., after infection or vaccination). In passive immunity, ready-made antibodies are directly introduced into the body (e.g., from mother to foetus via placenta).

• Speed of Response: Active immunity is slow and takes time to develop a full response. Passive immunity provides immediate relief and is very fast.

• Duration of Immunity: Active immunity is long-lasting and often lifelong because it involves the formation of memory cells (B and T-lymphocytes). Passive immunity is temporary and short-lived as the antibodies degrade over time and no memory cells are formed.

• Side Effects: Active immunity generally has no major side effects. Passive immunity can sometimes cause hypersensitivity reactions.

For the 2025-26 session, expect both direct and application-based questions from Cancer and AIDS, typically for 2 or 3 marks.

• For Cancer: You could be asked to differentiate between benign and malignant tumours, or to explain the property of metastasis. Questions on carcinogens (cancer-causing agents) and methods of detection (like biopsy) are also common.

• For AIDS: Important questions include identifying the causative agent (HIV), the target cells in the body (helper T-lymphocytes), and the mode of transmission. A question explaining why AIDS is considered a syndrome and not just a disease is a probable HOTS question. The principle of the ELISA test for diagnosis is also a key area.

This phenomenon, where the immune system attacks the body's own cells, is known as autoimmunity. It is considered a conceptual and high-order thinking question. The primary reason for this failure is a loss of immunological tolerance. This can happen due to:

• Genetic Factors: A genetic predisposition can make an individual more susceptible to developing an autoimmune disorder.

• Environmental Triggers: An infection by a virus or bacteria can sometimes trigger an autoimmune response. The pathogen's antigens may mimic the body's own proteins (a concept called molecular mimicry), causing the immune system to attack both the pathogen and the similar-looking body cells.

A classic example required in an answer is Rheumatoid Arthritis, where the immune system attacks the synovial membranes of joints, causing severe inflammation and damage.

A question on drug and alcohol abuse is often designed to test your understanding of its social and biological implications. For a high-scoring answer, ensure you cover these points:

• Causes: Mention curiosity, peer pressure, academic or social stress, and the perception of it being 'cool' as primary reasons for initiation.

• Concepts of Addiction and Dependence: Define addiction as a psychological attachment and dependence as the tendency of the body to manifest withdrawal symptoms if the substance is discontinued.

• Effects: List both short-term effects (reckless behaviour, vandalism) and long-term effects (damage to the nervous system, liver cirrhosis, depression). Mention social effects like poor academic performance and isolation.

• Prevention and Control: Conclude with measures like avoiding peer pressure, seeking parental help, and professional counselling. Mentioning the role of education is key.

This question assesses your understanding of the mechanisms behind active and passive immunity. The key difference lies in the generation of immunological memory.

• A vaccine contains weakened or inactivated pathogens or their antigens. When introduced into the body, it stimulates a primary immune response. This leads to the production of antibodies and, more importantly, B and T memory cells. These memory cells remain in the body for years. Upon a subsequent, actual infection, they recognize the antigen and mount a massive, rapid secondary immune response, preventing the disease. This is a form of artificially acquired active immunity.

• An antitoxin injection (like for tetanus or a snake bite) contains pre-formed antibodies. It provides immediate neutralisation of the antigen/toxin. However, it does not stimulate the recipient's immune system to produce its own antibodies or memory cells. The donated antibodies are eventually broken down and eliminated from the body. This is a form of artificially acquired passive immunity and is therefore short-lived.

This is a conceptual question that tests your understanding of the modes of transmission, a critical topic in this chapter. The success of a pathogen is directly linked to its mode of transmission:

• Common Cold (Rhinovirus): This is an airborne disease. It is transmitted through droplets released when an infected person coughs or sneezes. It can also spread through contact with contaminated surfaces (fomites). This ease of transmission allows it to infect a large number of people quickly, ensuring its survival and spread.

• AIDS (HIV): The Human Immunodeficiency Virus is transmitted only through specific means involving the exchange of body fluids. The primary modes are sexual contact with an infected person, sharing of contaminated needles, transfusion of infected blood, or from an infected mother to her child through the placenta. Because transmission requires such intimate or specific contact, its spread is much more restricted compared to airborne pathogens.