KEY POINTS

Antigens

1. Antigens (or immunogens) stimulate immune cells to induce an immune response.

2. The degree of immune response depends on several characteristics of the antigen.

Body's Defense Against Invading Antigens

1. Both nonimmune and immune mechanisms defend against invading antigens.

2. A first line of defense includes the skin and certain external and internal body fluids.

3. A second line of defense consists of phagocytic cells of the myeloid and macrophage-monocyte lineages.

4. Macrophage-derived cytokines can induce a variety of physiological processes to help combat infectious antigens.