Innate Immunity

Innate immunity is the body's built-in, non-specific system of defence. It is called "innate" because a person is born with it, and unlike acquired immunity it does not need any prior contact with a particular pathogen to work — it responds in more or less the same way to any invader, whether the body has met it before or not, and it acts immediately or within hours rather than days.

Innate immunity operates through four broad kinds of barriers working together:

Physical barriers are the body's outermost defences. The skin is the single largest physical barrier, forming an unbroken covering that most microbes simply cannot cross. Mucus secreted by the membranes lining the respiratory, gastrointestinal, and urogenital tracts traps invading microbes before they can penetrate deeper tissue.

Physiological barriers use the body's own chemistry against invaders. The strongly acidic environment of the stomach destroys most swallowed microorganisms, saliva in the mouth and tears from the eyes contain an antimicrobial enzyme called lysozyme that breaks down bacterial cell walls, and nasal hair filters incoming air of dust and some microbes.

Cellular barriers consist of white blood cells (leukocytes) that seek out and destroy pathogens. Polymorphonuclear leukocytes, particularly neutrophils, along with monocytes and macrophages, engulf and digest invading microbes in a process called phagocytosis. Natural killer (NK) lymphocytes destroy the body's own virus-infected cells and some tumour cells. Eosinophils are especially important against larger parasites, such as the helminth worms discussed later in this chapter.