Immunity

Chapter Overview

The immune system protects the body against infection through three sequential lines of defence. The first line of defence consists of physical and chemical barriers, primarily the skin (providing an impenetrable keratinized barrier) and the protective epithelial linings of the digestive and respiratory tracts, which secrete mucus and antimicrobial substances to trap and destroy invading microbes.

If pathogens breach these barriers, the second line of defence (nonspecific internal defence) is activated. This includes phagocytic blood cells (like neutrophils and macrophages), protective complement proteins that destroy bacterial cell membranes, the inflammatory response (causing vasodilation and increased vascular permeability), and the temperature response (fever) to inhibit pathogen growth and accelerate cell repair.

The third line of defence (specific immune response) adapts to target specific antigens. Monocytes mature into antigen-presenting cells to trigger lymphocytes. T cells coordinate the cell-mediated immune response, directly attacking infected cells. B cells coordinate the humoral (antibody-mediated) immune response, producing specific antibodies. Acquired immunity can be natural or artificial, and active or passive. However, immune dysfunction can lead to disorders like allergies, autoimmune diseases, or transplant rejection mediated by T and B cells. Understanding these immunology concepts is essential for HSSC-II SLO-based assessments.