Immune responses are directed at a series of foreign substances known as antigens, also referred to as immunogens. Most antigens are high molecular weight substances, but low molecular weight substances will also act as antigens if they bind to proteins in the body. The low molecular weight compound is referred to as a hapten. The resulting conjugate may induce an immune response directed against the antigen.

The uptake and processing of antigens by macrophages in the tissue is an initial, critical step in most immune responses. The simple act of taking foreign substances into the body does not necessarily invoke an immune response because the substances may be broken down before they are ingested by macrophages.

Antigenic determinants. The chemical groups on the antigen molecules that determine their immunogenicity are called antigenic determinants, also known as epitopes. Antigenic determinants may consist of several amino acids of a protein molecule or several monosaccharide units of a polysaccharide. Each species of living thing is chemically and antigenically unique because of differences in its proteins, carbohydrates, and other organic substances.

Types of antigens. Certain types of antigens are distinctive. Autoantigens, for example, are a person's own self antigens. Alloantigens are antigens found in different members of the same species (the red blood cell antigens A and B are examples). Heterophile antigens are identical antigens found in the cells of different species.

A single organism such as a bacterium may contain a variety of proteins, carbohydrates, and other materials that provoke immune responses. Antigens found on the body cell are called somatic antigens. Antigens in the bacterial capsule are capsular antigens. Antigens of an organism's flagella are known as flagellar antigens (H antigens). Protein substances such as exotoxins are also antigenic.