The word “defense”(or “resistance,” as is also commonly used in the same context) is an inclusive term. Host defense therefore is the sum of all host attributes which mitigate the invasive nature of infectious agents. An overview of the host defense system is represented by Figure 1. As seen in this figure, the host is protected initially by the skin and mucous membranes which function as a bar-rier to unimpeded microbial penetration of the host. Although certain infectious agents may cause disease without penetration of these anatomical barriers, they nevertheless serve as efficient barriers against other organisms. In addition it should be noted that the antimicrobial function of some of these barriers is enhanced by various cellular and humoral factors which are present on the skin and mucosal surfaces.

When successful penetration of an anatomical barrier occurs, the invading microorganism will probably first encounter nonspecific host defenses. These are constitutive properties of the healthy host which require no previous experience with the invading organism for their presence in the host or their activity against the disease causing agent. These nonspecific defenses are also referred to as the host's innate immunity. After infectious challenge the specific host de-fense mechanisms are activated and active cellular and/or humoral immunity is established. While it is frequently true that an infectious agent may encounter the nonspecific defenses first and will later be confronted with specific immunity after the host has had sufficient time to develop an immune response, exposure to a particular infectious agent may represent a second or third exposure. If this is the case, specific defenses will be operative and the infectious agent will be faced