Immunity

Chapter Overview

Immunity is a crucial medical physiology chapter in Class 12 Biology exploring the biological systems that defend the body against pathogenic microorganisms, toxins, and abnormal cells. The defense apparatus is stratified into three progressive lines of defence. The first line encompasses physical and biochemical barriers: stratified keratinized epidermis of the skin, bactericidal sebaceous and sweat secretions, ciliary escalators and mucus-trapping epithelial linings of the respiratory system, and stomach gastric acid and salivary lysozyme in the digestive tract that neutralize pathogens at portal entry points.

When pathogens breach surface barriers, the innate second line of defence responds rapidly. Phagocytic white blood cells, notably neutrophils and macrophages, engulf and digest microbes via phagolysosomal pathways, while natural killer (NK) cells induce apoptosis in virus-infected and cancerous cells. Protective humoral proteins reinforce this cellular defense: the complement system cascades to lyse foreign cell walls and promote opsonization, while antiviral interferons inhibit viral protein translation in adjacent cells. The localized inflammatory response-mediated by histamine and prostaglandins-increases vascular permeability to recruit leukocytes, coupled with systemic pyrogen-induced fever.

The third line of defence provides acquired, highly specific adaptive immunity characterized by antigen recognition and immunological memory. The chapter contrasts cell-mediated immunity executed by cytotoxic T cells (CD8+) and helper T cells (CD4+) with antibody-mediated (humoral) immunity executed by B lymphocytes differentiating into antibody-secreting plasma cells. The molecular structure of immunoglobulins and clonal selection theory are thoroughly analyzed. Finally, the chapter evaluates clinical malfunctions: type I hypersensitivities (allergies), loss of self-tolerance in autoimmune disorders, and immune mechanisms of tissue allograft rejection.