Immune System

Innate and adaptive immunity: antigens, antibodies, and immunological memory

The immune system has two main branches. Innate immunity provides immediate, non-specific defense through barriers (skin, mucous membranes), phagocytes (macrophages, neutrophils), and inflammation. Adaptive immunity is antigen-specific and develops over days. When antigen-presenting cells (APCs) display pathogen fragments, helper T cells (CD4+) activate. B cells that recognize the antigen differentiate into plasma cells (producing antibodies) and memory B cells. Cytotoxic T cells (CD8+) kill infected host cells. Antibodies (immunoglobulins) bind antigens via complementarity-determining regions, neutralizing pathogens or marking them for destruction (opsonization). Upon re-exposure, memory cells mount a secondary response that is faster (1–3 days vs 7–14 days) and produces 10–100× more antibodies. Vaccination exploits this by introducing weakened or inactivated antigens to generate memory without disease.