Chapter 7: Human Health and Disease | Summary

Immunity is the host's overall ability to fight disease-causing organisms and is of two types: innate and acquired. Innate immunity is non-specific, present from birth, and operates through four barriers: physical (skin, mucus), physiological (stomach acid, saliva, tears), cellular (neutrophils, monocytes, macrophages, NK cells) and cytokine (interferons from virus-infected cells). Acquired immunity is pathogen-specific and characterised by immunological memory, generating a stronger secondary (anamnestic) response on repeat exposure to the same pathogen.

Acquired immunity involves B-lymphocytes producing antibodies (humoral immunity, H2L2 structure with IgA, IgM, IgE, IgG classes) and T-lymphocytes mediating cell-mediated immunity (CMI). Active immunity develops following natural infection or vaccination; passive immunity involves transfer of ready-made antibodies, as seen in colostrum (IgA) and placental transfer. Vaccination exploits immunological memory to protect against future infections; recombinant DNA technology has enabled production of safer vaccines such as hepatitis B vaccine from yeast.

Allergy is the exaggerated immune response to environmental antigens called allergens (dust mites, pollens, animal dander). The antibodies involved are of IgE type, and symptoms—sneezing, watery eyes, breathing difficulty—arise due to release of histamine and serotonin from mast cells. Anti-histamines, adrenalin and steroids are used to reduce allergic symptoms.

Auto-immune disease occurs when the immune system mistakenly attacks self-cells due to genetic or unknown reasons, causing damage to the body. Rheumatoid arthritis is a well-known auto-immune disease. The immune system also plays roles in organ transplant rejection; tissue and blood group matching, along with life-long immuno-suppressants, are required to prevent graft rejection.

The human immune system consists of lymphoid organs, tissues, cells and soluble molecules. Primary lymphoid organs—bone marrow and thymus—are sites where lymphocytes mature. The thymus, located near the heart, is large at birth and reduces in size by puberty. Secondary lymphoid organs include spleen, lymph nodes, tonsils, Peyer's patches and appendix, which provide sites for antigen–lymphocyte interaction.

The spleen filters blood-borne microorganisms and has a reservoir of erythrocytes. Lymph nodes trap antigens from lymph and tissue fluid, activating lymphocytes. Mucosa-associated lymphoid tissue (MALT), lining the respiratory, digestive and urogenital tracts, constitutes about 50 per cent of lymphoid tissue in the human body.

AIDS (Acquired Immuno Deficiency Syndrome) is caused by HIV, a retrovirus with an RNA genome enclosed in a protein envelope. First reported in 1981, it has killed more than 25 million people worldwide. Transmission occurs through sexual contact with an infected person, transfusion of contaminated blood, sharing infected needles, or from an infected mother to child through the placenta.

HIV infects macrophages and helper T-lymphocytes (TH). Using reverse transcriptase, viral RNA is converted to DNA, which integrates into host cell DNA. Progressive destruction of TH cells renders the patient unable to fight even minor infections (opportunistic infections by Mycobacterium, fungi, Toxoplasma). Diagnosis uses ELISA; anti-retroviral drugs prolong life but do not cure the disease. Prevention involves safe sex, use of disposable needles, screening blood supplies and awareness programmes by NACO and WHO.

Cancer arises when the normal regulatory mechanisms controlling cell growth and differentiation break down. Normal cells exhibit contact inhibition; cancer cells lose this property, dividing uncontrollably to form tumors. Benign tumors remain localised; malignant tumors invade surrounding tissues and spread via blood to distant sites—a process called metastasis. Carcinogens include ionising radiations (X-rays, gamma rays), UV radiation, chemical carcinogens (tobacco smoke) and oncogenic viruses carrying viral oncogenes.

Cancer is detected by biopsy, histopathological studies, blood/bone marrow tests, radiography, CT and MRI scans, and antibodies against cancer-specific antigens. Treatment involves surgery, radiation therapy, chemotherapy and immunotherapy (biological response modifiers such as α-interferon). Early detection is crucial for successful treatment.

Commonly abused drugs include opioids (heroin/smack from Papaver somniferum), cannabinoids (marijuana, hashish, charas, ganja from Cannabis sativa) and coca alkaloids (cocaine from Erythroxylum coca). Heroin is a depressant; cocaine is a stimulant causing euphoria; cannabinoids affect the cardiovascular system. Tobacco contains nicotine, which stimulates adrenaline and nor-adrenaline release, raising blood pressure and heart rate; smoking is linked to lung, bladder and throat cancers.

Addiction is psychological attachment to effects such as euphoria; dependence involves physical withdrawal symptoms (anxiety, nausea, shakiness) on discontinuation. Adolescence (12–18 years) is a particularly vulnerable phase due to curiosity, peer pressure and stress. Harmful effects include reckless behaviour, coma, organ damage, HIV/hepatitis B risk (intravenous users), and hormonal side-effects from anabolic steroids. Prevention involves education, counselling, parental involvement and professional de-addiction programmes.

Chapter 7 establishes that human health is a multidimensional concept—encompassing physical, mental and social well-being—and that diseases arise from genetic disorders, infections or unhealthy lifestyles. The immune system, with its innate and acquired arms, forms our primary defence; vaccination harnesses its memory to prevent infectious diseases. AIDS and cancer, though caused by very different mechanisms, share the common theme of compromised cellular control and require early detection and preventive action for effective management.

From a board examination perspective, this chapter is highly significant as questions regularly appear on the life cycle of Plasmodium, structure of antibodies, types of immunity, HIV replication mechanism, cancer versus normal cell behaviour, and effects of drug abuse. Understanding key terms such as metastasis, haemozoin, MALT, ELISA, carcinogen and allergen, along with the ability to compare innate versus acquired and active versus passive immunity, is essential for scoring well in CBSE Class 12 Biology.