Cellular Barriers and Immune Cells — NEET Biology

Cellular barriers involve leukocytes like PMNLs (neutrophils), monocytes, and NK cells that provide non-specific defense by engulfing pathogens and infected cells. Students often confuse these cellular barriers with chemical barriers (lysozyme, HCl) or humoral immunity, forgetting that these WBCs act as physical/cellular defense mechanisms. The key is remembering that PMNLs perform phagocytosis of bacteria, monocytes become macrophages for antigen presentation, and NK cells destroy virus-infected cells—all without prior antigen exposure. Since immune barriers are fundamental to understanding disease resistance, NTA tests the distinction between these three cell types and their specific roles in innate immunity.