CBSE Class 12 Biology: Human Health & Disease — Notes 2026

Immunity Types

• Innate immunity: non-specific; present from birth; first and second lines of defence
• Acquired (adaptive) immunity: specific; develops after exposure; humoral (antibody-mediated) and cell-mediated
• Active immunity: host produces own antibodies (after infection or vaccination); passive immunity: ready-made antibodies (mother's milk, antivenom)

Lymphoid Organs

• Primary: bone marrow (B cell maturation, all blood cells origin); thymus (T cell maturation, shrinks after puberty)
• Secondary: spleen (filter blood; immune response), lymph nodes (filter lymph; B and T cells interact with antigen), tonsils, Peyer's patches (gut immunity)
• MALT (Mucosa-Associated Lymphoid Tissue): 50% of lymphoid tissue; in respiratory, digestive, urogenital tracts

Immune Response

• Antigen: substance triggering immune response; usually protein on pathogen surface
• B lymphocytes: produce antibodies (immunoglobulins); 5 classes (IgG, IgA, IgM, IgE, IgD)
• T lymphocytes: Helper T (CD4+, coordinate response), Cytotoxic T (CD8+, kill infected cells), Memory T cells (rapid response to re-exposure)

AIDS (Acquired Immunodeficiency Syndrome)

• HIV (Human Immunodeficiency Virus): retrovirus; RNA genome; reverse transcriptase
• HIV infects Helper T (CD4+) cells; reduces CD4+ count; immune system collapses; opportunistic infections follow
• Transmission: sexual contact, contaminated blood/needles, mother to child (vertically); NOT through casual contact

Cancer

• Uncontrolled cell division due to mutation in proto-oncogenes (→ oncogenes) or tumour suppressor genes (p53)
• Types: carcinoma (epithelial tissue), sarcoma (connective tissue), leukemia (blood), lymphoma (lymphoid tissue)
• Metastasis: cancer cells spread to distant sites via blood/lymph; forms secondary tumours; makes treatment difficult

Malaria

• Causative agent: Plasmodium (P. vivax, P. falciparum — most dangerous)
• Vector: female Anopheles mosquito; lifecycle: sporozoites in saliva → liver (exo-erythrocytic) → RBCs (erythrocytic)
• Prevention: mosquito nets, DDT (banned), indoor residual spraying; treatment: chloroquine (resistance developing), artemisinin

CBSE Board Focus

• Health and Disease: 6–8 marks; types of immunity, AIDS mechanism, cancer definition and types
• Draw antibody structure (Y-shaped with 4 chains, 2 heavy + 2 light; Fab and Fc regions)
• Malaria lifecycle: two hosts (mosquito + human); two phases — liver phase and blood phase