Immunity: Your Body's Multi-Layered Defense System Against Pathogens

Immunity is the integrated capacity to recognize, neutralize, and remember harmful agents (pathogens, toxins, cancer cells).

Two Arms

Key Organs & Tissues

• Primary lymphoid: Bone marrow (hematopoiesis), Thymus (T-cell maturation).
• Secondary lymphoid: Lymph nodes, spleen, MALT (tonsils, Peyer's patches, appendix) – sites of antigen encounter.

Vaccination Principle

Mimics infection → generates memory without disease. Herd immunity protects non-immune individuals.

Clinical Relevance

• Immunodeficiency: Primary (genetic) or secondary (HIV, chemo, steroids) → recurrent/severe infections.
• Autoimmunity: Loss of self-tolerance (RA, SLE, T1DM).
• Hypersensitivity: Type I (allergy), II (antibody-mediated), III (immune complex), IV (T-cell delayed).

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Innate (Non-specific, Immediate)	Adaptive (Specific, Delayed, Memory)
Physical barriers (skin, mucosa, cilia)	B cells → antibodies (humoral)
Phagocytes: neutrophils, macrophages	T cells: CD8+ cytotoxic, CD4+ helper (Th1/Th2/Th17/Tfh/Treg)
NK cells	Memory B/T cells → faster, stronger secondary response
Complement, cytokines, interferons	Somatic hypermutation, class switching
Inflammation (acute)	Immunological memory (years to lifetime)