Revision: Biology and Human Welfare >> Human Health and Diseases Biology Science (English Medium) Class 12 CBSE

Health is defined as the state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.

The overall ability of the host to fight the disease-causing organisms conferred by the immune system is called immunity.

Infection is the entry, development, or multiplication of an infectious agent in the human body or animals.

Antibiotics are metabolic products of microorganisms whose very low concentrations are inhibitory or detrimental to other microbes.

A preparation containing specific antigens that is administered to induce temporary or permanent immunity against a particular disease is called a vaccine.

Preparation of antigenic proteins of pathogens (weakened or killed) which on inoculation into a healthy person provides temporary/permanent immunity against a particular disease.

The process of administering a vaccine to stimulate the immune system and provide protection against infectious diseases is called vaccination.

Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity of the immune system to something in the environment that usually causes no adverse effects to most people. These diseases include hay fever, dermatitis, asthma, food allergies, etc.

The exaggerated response of the immune system to certain antigens present in the environment is called allergy.

Autoimmune response is an immune reaction in which antibodies or immune cells are produced against the body’s own antigens.

Autoimmune disease is a pathological condition caused due to autoimmunity, where self-tissues are damaged or destroyed by the immune system.

Autoimmunity is the condition in which the immune system fails to recognise self and reacts against the body’s own cells and tissues.

The immunity developed during an individual’s lifetime after exposure to antigens is called acquired (specific) immunity.

The antibody-mediated defence mechanism is called humoral immune response.

The defence mechanism involving lymphocytes and macrophages without antibody production is called cell-mediated (cellular) immune response.

The system of defence mechanisms that protects the body against harmful agents is called immune system.

The hereditary, non-specific protection present from birth is called innate (natural) immunity.

Tumour is a mass of abnormally proliferating cells formed due to uncontrolled cell division.

Metastasis is the process by which cancer cells spread from the primary site to distant organs through blood or lymph.

Cancer is a disease characterised by uncontrolled and abnormal division of body cells due to loss of normal growth regulation.

Benign tumour is a non-cancerous tumour that remains confined to its original site and does not spread to other parts of the body.

Malignant tumour is a cancerous tumour that grows rapidly, invades surrounding tissues and spreads to distant organs.

Agents that cause cancer by disturbing the normal genetic and regulatory processes of cells are called carcinogens.

A cancer-causing gene that induces transformation of normal cells into cancerous cells is called an oncogene.

A normal cellular gene involved in growth and development, which can become an oncogene after mutation or activation, is called a proto-oncogene.

Adverse functioning of one or more organs or systems of the body, characterised by specific signs and symptoms, is called disease.

A state of complete physical, mental and social well-being and not merely the absence of disease or infirmity is called health.

The condition in which the immune system fails to distinguish self from non-self and attacks the body’s own cells is called as autoimmunity.

The substance that triggers an allergic immune response in a sensitive individual is called as allergen.

The exaggerated or hypersensitive immune response of the body to certain harmless environmental substances is called as allergy.

The process of providing immediate protection by injecting preformed antibodies into the body is called as passive immunisation.

The process by which an individual acquires protection against a specific disease through vaccination or antibody administration is called as immunisation.

The process of introducing weakened or inactivated pathogens or their antigenic components into the body to induce immunity is called as vaccination.

Innate immunity is the inborn, non-specific defence that protects the body from pathogens from birth.

The immunity developed when the body produces its own antibodies in response to exposure to antigens through infection or vaccination is called as active immunity.

The immunity obtained by the direct transfer of ready-made antibodies from another individual is called as passive immunity.

Organisms such as bacteria, viruses, fungi, protozoans and helminths that cause diseases in humans are called pathogens.

Diseases that do not spread from one person to another and arise due to genetic factors, lifestyle or physiological disorders are called non-infectious diseases.

Diseases that are easily transmitted from one person to another are called infectious diseases.

• WHO Health = Complete physical, mental and social well-being; not merely absence of disease.
• Immunity = Ability of the host to fight disease via the immune system; distinguishes self from non-self.
• Antigen = foreign substance stimulating an immune response; Antibody = protective chemical produced against antigens.
• Concept started by Edward Jenner — developed cowpox vaccine against smallpox. Study of the immune system = Immunology.
• Memory Cells — Formed after infection; live for years; on second invasion → rapidly multiply → faster, stronger response → explains why childhood diseases are not contracted twice.

• Meaning: Non-specific, hereditary defence present from birth; not affected by prior infection or vaccination.
• Physical Barriers: Skin, mucus lining, nasal hair, cough reflex, and flushing action of urine/tears block microbial entry.
• Physiological Barriers: Stomach acid, saliva, and tears (containing Lysozyme) prevent microbial growth.
• Cellular Barriers: Neutrophils, monocytes, and macrophages destroy microbes via phagocytosis; NK cells fight viruses and tumours.
• Cytokine Barriers: Infected cells release interferons to protect neighbouring cells from viral infection.
• Blood Proteins: The complement system (30+ proteins) eliminates pathogens; Acute Phase Proteins (CRP, etc.) surge during infection to aid repair and resistance.

• Meaning: Immunity developed during lifetime; also called Adaptive/Specific immunity. Works by producing antibodies against specific antigens.
• Key Features: Specificity, Diversity, Self vs Non-self discrimination, and Memory (faster response on second exposure to the same pathogen).
• Active Immunity: The body produces its own antibodies. Natural — after infection (e.g., measles, lifelong). Artificial — via vaccines (e.g., Polio, BCG).
• Passive Immunity: Ready-made antibodies received externally. Natural — from mother via placenta/colostrum. Artificial — injected serum from immunised animals (e.g., anti-rabies from horses).
• Key Difference: Active immunity is long-lasting (memory cells formed); Passive immunity is short-lived (no memory cells).

• Principle: Works on the immune system's memory. A vaccine (weakened/inactivated pathogen or its proteins) stimulates antibody production and generates memory B and T-cells.
• On Re-exposure: Memory cells quickly recognise the pathogen and produce massive antibodies to neutralise it.
• Passive Immunisation: Preformed antibodies/antitoxins are directly injected when quick response is needed (e.g., tetanus, snakebite).
• Types of Vaccines: Protein/sugar from pathogen, dead/inactivated pathogen, toxoid, or attenuated (weakened) pathogen.
• Recombinant DNA Vaccines: Antigenic polypeptides produced in bacteria/yeast for large-scale production (e.g., Hepatitis B vaccine from yeast).

• Chemicals: Nicotine, caffeine, polycyclic hydrocarbons, coal/oil combustion products, and excess sex hormones are carcinogenic (e.g., breast cancer linked to hormones).
• Radiation: X-rays, gamma rays, UV rays, and cosmic rays cause cancer. High UV exposure increases the risk of skin cancer.
• Viruses: Oncogenic viruses carry v-onc genes (e.g., EBV, HPV).
• Oncogenes: Normal cells contain proto-oncogenes (c-onc genes), which, when activated, transform cells into cancerous ones.
• Addiction: Tobacco → mouth, lips, lung cancer; Alcohol → oesophagus, liver cancer; Drugs (marijuana, steroids) → also cause cancer.

• Dependence: Repeated use of drugs/alcohol creates a psychological attachment. Stopping suddenly causes withdrawal syndrome.
• Immediate Effects: Reckless behaviour, violence, heart failure, cerebral haemorrhage — may lead to coma and death.
• Long-term Effects: Mental and financial distress to the addict and family. Chronic use causes nervous system damage and liver cirrhosis. Alcohol use in adolescence may lead to heavy drinking in adulthood.
• Prevention Measures: Avoid undue peer pressure, seek education and counselling, maintain open communication with parents, and look out for danger signs early.
• Medical Help: Professional and medical help should be sought immediately upon identifying signs of drug/alcohol abuse — early intervention improves recovery.

• The immune system consists of lymphoid organs, tissues, cells and soluble molecules like antibodies that protect the body against foreign antigens.
• It has the unique ability to recognise antigens, respond to them and retain memory for faster and stronger responses on repeated exposure.
• Primary lymphoid organs (bone marrow and thymus) are responsible for the origin, development and maturation of lymphocytes.
• Secondary lymphoid organs such as spleen, lymph nodes, tonsils, Peyer’s patches and appendix are sites where lymphocytes interact with antigens and initiate immune responses.
• Mucosa-associated lymphoid tissue (MALT), present in respiratory, digestive and urogenital tracts, forms nearly 50% of the body’s lymphoid tissue and provides defence at body surfaces.