Innate Immunity

Innate immunity is the body's first line of defence — it is present from birth and works against any foreign agent, regardless of what it is. Unlike adaptive immunity, it does not 'remember' a specific pathogen. Instead, it relies on a set of ready-made barriers that block or destroy invaders before they can cause harm.

The textbook describes four types of barriers that together make up innate immunity.

Physical barriers are the body's outermost walls. The skin is the main physical barrier — its tough, dry surface prevents most microorganisms from entering. Inside the body, the mucous membranes lining the respiratory tract, the gastrointestinal tract, and the urogenital tract secrete mucus. This sticky coating traps microbes that land on these surfaces, preventing them from reaching deeper tissues.

Physiological barriers work by creating conditions that are hostile to microbial growth. The hydrochloric acid in the stomach kills many swallowed bacteria. Saliva in the mouth contains enzymes that break down bacterial cell walls. Tears from the eyes wash away microbes and also contain lysozyme, an enzyme that destroys bacterial cell walls.

Cellular barriers involve specialised white blood cells that actively destroy invaders. Several types of leukocytes (WBCs) carry out this role:

• Polymorphonuclear leukocytes (PMNLs or neutrophils) — these are the most abundant type of white blood cell and are rapid responders to infection.
• Monocytes — these circulate in the blood and later mature into macrophages once they enter tissues.
• Natural killer (NK) cells — a type of lymphocyte that can kill virus-infected cells and tumour cells.
• Macrophages — large cells found in tissues that engulf and digest microbes through phagocytosis.

All of these cells can phagocytose (engulf and destroy) microbes, providing a direct cellular attack on pathogens. …