CBSE Class 12 Biology Revision Notes: Human Health and Disease

Key Terms & Definitions

Pathogen

Any disease-causing microorganism or agent, such as bacteria, viruses, fungi, protozoans, or worms.

Immunity

The ability of an organism to resist infection by pathogens or the toxins produced by them.

Vaccination

The process of introducing a weakened or inactivated pathogen (vaccine) into the body to stimulate an immune response and provide immunity.

Allergy

An exaggerated response of the immune system to certain antigens present in the environment (allergens), such as pollen or dust mites.

Autoimmunity

A condition where the immune system mistakenly attacks and destroys healthy body tissues, perceiving them as foreign.

Carcinogen

Any agent, substance, or radiation that can cause cancer by damaging DNA or disrupting cellular processes.

Metastasis

The process by which cancer cells spread from their primary site to other parts of the body, forming secondary tumors.

Addiction

A state of physiological and psychological dependence on certain substances (drugs/alcohol) that leads to compulsive drug-seeking behavior despite harmful consequences.

Understanding Immunity: Your Body's Defense System

Immunity is the body's ability to defend itself against disease-causing organisms. It's broadly classified into two main types: Innate Immunity and Acquired Immunity.

Innate Immunity is non-specific and present from birth, offering a general defense. It consists of various barriers:

• Physical Barriers: Skin (prevents entry of microbes), mucous coating of respiratory, gastrointestinal, and urogenital tracts (traps microbes).
• Physiological Barriers: Acid in the stomach, saliva in the mouth, tears from eyes (all prevent microbial growth).
• Cellular Barriers: Phagocytic cells like neutrophils, macrophages, and natural killer (NK) cells (engulf and destroy microbes).
• Cytokine Barriers: Virus-infected cells secrete proteins called interferons, which protect non-infected cells from further viral infection.

Acquired Immunity is pathogen-specific and is developed or acquired during an individual's lifetime. It's characterized by memory, meaning the immune system remembers previous encounters with pathogens and mounts a stronger, faster response.

It can be further categorized:

• Humoral Immune Response (Antibody-Mediated Immunity): Mediated by B-lymphocytes, which produce antibodies—proteins that target and neutralize pathogens or their toxins circulating in body fluids.
• Cell-Mediated Immune Response (CMI): Mediated by T-lymphocytes, which directly attack infected cells or cancer cells. T-cells are also responsible for graft rejection during organ transplantation.

Acquired immunity can also be:

• Active Immunity: When an individual's own body produces antibodies in response to an infection or vaccination. It is slow to develop but long-lasting.
• Passive Immunity: When pre-formed antibodies are directly given to protect the body, such as maternal antibodies transferred to a fetus (via placenta) or infant (via colostrum). It provides immediate protection but is short-lived.

Must Remember: Common Diseases and Their Characteristics

• Typhoid: Caused by Salmonella typhi, transmitted via contaminated food/water. Symptoms: high fever, weakness, stomach pain, constipation. Diagnosed by Widal test.
• Pneumonia: Caused by Streptococcus pneumoniae and Haemophilus influenzae, affects lungs (alveoli fill with fluid). Transmission: airborne droplets. Symptoms: fever, chills, cough, headache.
• Common Cold: Caused by Rhinoviruses, affects nose and respiratory passage. Transmission: airborne droplets, contaminated objects. Symptoms: nasal congestion, sore throat, cough, headache.
• Malaria: Caused by Plasmodium (protozoan), transmitted by female Anopheles mosquito. Life cycle involves humans and mosquitoes. Symptoms: recurring high fever, chills.
• Amoebiasis (Amoebic Dysentery): Caused by Entamoeba histolytica, transmitted via contaminated food/water (houseflies act as mechanical carriers). Symptoms: constipation, abdominal pain, cramps, blood/mucus in stools.
• Ascariasis: Caused by Ascaris lumbricoides (roundworm), transmitted via contaminated food/water/soil. Symptoms: internal bleeding, muscular pain, fever, anaemia, blockage of intestinal passage.
• Filariasis (Elephantiasis): Caused by Wuchereria bancrofti and W. malayi (filarial worms), transmitted by mosquito bites. Symptoms: chronic inflammation of lymphatic vessels of lower limbs, leading to gross deformities.
• Ringworm: Caused by fungi like Microsporum, Trichophyton, Epidermophyton. Symptoms: dry, scaly lesions on skin, nails, scalp, accompanied by intense itching. Spreads via infected clothes, towels.

Psychoactive Drugs: Categories and Effects

Exam Tip: Scoring in Human Health and Disease

To excel in this chapter, focus on diagrams and life cycles – especially that of Plasmodium (Malaria) and the antibody structure. Pay close attention to the modes of transmission and preventive measures for common diseases, as these are frequent targets for 3-5 mark questions. Differentiate clearly between innate and acquired immunity, and between active and passive immunity. Understand the various carcinogens and types of cancer, along with their detection and treatment. For drug abuse, know the source plant, chemical name, and physiological effects of common drugs. Practice drawing neat, labeled diagrams for disease cycles and antibody structure to fetch full marks.

Quick Examples: Key Concepts Illustrated

• {"title":"1. Malaria Life Cycle","description":"Example: Plasmodium vivax (malarial parasite) completes its life cycle in two hosts: female Anopheles mosquito (sexual phase) and humans (asexual phase). The infectious stage for humans is sporozoites, which are injected into the bloodstream, travel to the liver, then to RBCs, causing cycles of fever and chills. Mosquitoes become infected by ingesting gametocytes from an infected human."}
• {"title":"2. HIV/AIDS Infection","description":"Example: The Human Immunodeficiency Virus (HIV) is a retrovirus that attacks helper T-lymphocytes (CD4+ T-cells), which are crucial for acquired immunity. By reducing the number of helper T-cells, HIV severely weakens the immune system, leading to Acquired ImmunoDeficiency Syndrome (AIDS), making the individual susceptible to various opportunistic infections."}
• {"title":"3. Cancer and Carcinogens","description":"Example: Tobacco smoke contains numerous carcinogens (e.g., polycyclic aromatic hydrocarbons) that can cause lung cancer. Ionizing radiations (X-rays, gamma rays) and non-ionizing radiations (UV rays) are also carcinogens, causing DNA damage that can lead to uncontrolled cell proliferation and tumor formation."}

Practice Questions with Solutions

• Q: Differentiate between active and passive immunity. A: Active immunity is developed by the body's own immune system in response to an antigen (slow but long-lasting). Passive immunity involves the direct transfer of pre-formed antibodies (immediate but short-lived).
• Q: Name the pathogen and mode of transmission for Typhoid and Amoebiasis. A: Typhoid: Salmonella typhi, transmitted via contaminated food/water. Amoebiasis: Entamoeba histolytica, transmitted via contaminated food/water, houseflies act as carriers.
• Q: What is the primary target cell of HIV in the human body? A: HIV primarily targets and destroys helper T-lymphocytes (CD4+ T-cells), which are vital for coordinating the immune response.
• Q: Define 'metastasis' in the context of cancer. A: Metastasis is the property of malignant tumors where cancer cells detach from the primary tumor, travel through blood or lymph, and form secondary tumors at distant sites in the body.

Frequently Asked Questions