The immune system senses the presence of microbial invaders by pattern recognition. This ancient and evolved system of recognizing organisms that are “non-self” was vital to the early survival of animals. The patterns recognized by the innate immune response are called pathogen-associated molecular patterns, which are biochemical motifs that are not found in vertebrates. Inflammation is the result of this pattern recognition by the innate immune system. In contrast, the patterns recognized by the adaptive immune response are known as epitopes. These are usually subunits of large foreign proteins called antigens. B and T lymphocyte responses result from this pattern recognition, and the result is a more specific and targeted response to foreign invaders or non-self-antigens. These two primary systems are often engaged in a synergistic response to infections and other bodily insults that animals encounter throughout their lives. Likewise, knowledge of the adaptive immune system sets the stage for modern vaccines and other therapeutics used in veterinary practice.