Class 12 Biology Chapter 21 Immunity Notes | Punjab Board PDF

Class 12 Biology Chapter 21 Immunity provides a detailed examination of the human immune system and its defence strategies against pathogenic microorganisms. The body relies on three sequential lines of defence. The First Line of Defence consists of innate, non-specific physical and chemical barriers – including keratinized skin, lysozyme secretions, stomach acid, and mucus-secreting epithelial linings of the digestive and respiratory tracts.

The Second Line of Defence operates when pathogens breach external barriers. It features non-specific cellular components (macrophages, neutrophils, and Natural Killer cells), antimicrobial proteins (the complement system and viral-inhibiting interferons), the inflammatory response (vasodilation, histamine release, and phagocyte recruitment), and the systemic fever (temperature) response triggered by pyrogens.

The Third Line of Defence provides specific, adaptive immunity mediated by lymphocytes. Students differentiate between Cell-Mediated Immunity (driven by Cytotoxic and Helper T cells) and Antibody-Mediated (Humoral) Immunity (driven by B cells differentiating into antibody-secreting plasma cells). The chapter concludes with immune system malfunctions, including immediate and delayed hypersensitivities (allergies), autoimmune diseases (where self-antigens are attacked), and graft/transplant rejection. For pre-medical HSSC Part 2 students preparing for Punjab Board exams under Lahore, Rawalpindi, Gujranwala, Multan, and other boards, Chapter 21 is a high-yield topic. These notes follow the PCTB / PECTAA 2026-2027 syllabus with complete solved exercises, MCQs, short questions, and long questions.