The Immune System: Innate and Adaptive Defenses Against Pathogens

The immune system is a distributed network of cells, tissues, and molecules defending against infectious agents and malignant transformation.

Innate Immunity (Rapid, Non-specific, No Memory)

• Barriers: Skin, mucosal epithelium, ciliary escalator, low pH, lysozyme, defensins.
• Cellular: Neutrophils (phagocytosis, NETs), macrophages, dendritic cells, NK cells, γδ T cells, ILCs.
• Molecular: Complement (opsonization, lysis, chemotaxis), acute-phase proteins (CRP, MBL), cytokines (IFN-α/β, TNF, IL-1, IL-6).
• Pattern Recognition: TLRs, NLRs, RLRs detect PAMPs/DAMPs.

Adaptive Immunity (Slow, Specific, Memory)

• Humoral: B cells → plasma cells → antibodies (IgM, IgG, IgA, IgE, IgD). Neutralize, opsonize, activate complement, ADCC.
• Cell-Mediated: CD4⁺ Th1/Th2/Th17/Tfh/Treg; CD8⁺ CTLs (kill infected/cancerous cells via perforin/granzyme, FasL).
• Memory: Long-lived plasma cells, memory B/T cells → faster, stronger secondary response.

Integration

Dendritic cells bridge innate → adaptive (antigen presentation, costimulation, cytokine milieu). Trained immunity (epigenetic reprogramming of innate cells) blurs the dichotomy.