The immune system is a recognition system that distinguishes self from non-self. A foreign substance which elicits a proliferation of cells that either attack specific invaders directly or produce antibodies. The immune response must be primed by the presence of an antigen. Exposure to a particular foreign agent enhances future response by that same invader. Active immunity occurs when the body is stimulated to produce antibodies.
Passive immunity occurs when the body acquires injected antibodies.
A. Duality of the Immune System 1.The immune system has two components mediated by two different kinds of cell. Humoral immunity results in production of antibodies which circulate as soluble proteins in blood and lymph. Defends against free bacteria and viruses in body fluids. Cell-mediated immunity is carried out by specialized cells circulating in blood and lymph. Protects against bacteria and viruses that have already infected cells.