Introduction

The immune system has evolved to provide protection against invading pathogenic organisms, foreign cells and cancer cells. This chapter describes basic mechanisms involved in innate and acquired immune responses and describes the particular relevance for critical illness, notably sepsis and multiple organ dysfunction syndromes.

Innate immunity

Immunity is the state of protection from infectious disease or our own altered cells, and comprises both specific and nonspecific components. Nonspecific, or innate immunity, is the basic in-built resistance to disease and consists of four defensive barriers that offer protection through anatomical, physiological, phagocytic and inflammatory strategies.

Anatomical barriers

THE SKIN

Intact skin prevents the penetration of most pathogens. The thinner outer epidermis is renewed every 2 to 4 weeks and does not contain blood vessels. The thicker dermis is composed of connective tissue and contains blood vessels, hair follicles, sebaceous glands and sweat glands. The sebaceous glands produce the oily substance sebum, made up of lactic acid and fatty acids, which keeps the pH of the skin at around 4 to inhibit bacterial growth.

Breaks in the skin such as small cuts and insect bites are obvious routes of infection.

MUCOUS MEMBRANES

Many pathogens can enter the body by penetrating mucous membranes, although they are protected by saliva, tears and mucus, which wash away organisms and also contain antiviral and antibacterial substances.