Basic concepts of immunology

Immunology is the branch of biology devoted to understanding the immune system — the network of cells, tissues, organs and molecules that shields an organism from disease-causing agents in its environment. An organism's overall capacity to resist a pathogen is termed immunity, and it is sometimes called disease resistance; the opposite condition, where the body cannot mount an effective defence, is susceptibility.

A defining feature of a healthy immune system is specificity combined with self/non-self discrimination: immune reactions are meant to be destructive only toward molecules that are foreign to the body ('non-self'), while the body's own molecules ('self') are normally left untouched. Any molecule — commonly a protein, polysaccharide or nucleic acid — that is capable of triggering this kind of response when it is foreign to the recipient is called an antigen, a name literally built from 'ANTIbody GENerator' because such molecules provoke antibody formation.

When the distinguishing machinery misfires and the immune system attacks the body's own tissue, the result is autoimmune disease, which can be serious or even fatal — a theme the chapter returns to later. Broadly, the immune defences an organism deploys fall into two categories that organize everything that follows: innate immunity, present from birth, and acquired (adaptive) immunity, built up over an individual's lifetime in response to specific exposures.