forces are prepared to respond to attacks in the air, sea, or on land, depending on where the enemy threatens.

Pathogens, like enemy armies, have different modes of attack, and the immune system, like the military, has specialized divisions to fight back in the relevant mode.

Broadly speaking, pathogens invade the body in two modes. Some pathogens live in the body's fluids but remain outside the host cells. These are called extracellular pathogens. Other pathogens are intracellular. Intracellular pathogens spend at least part of their life cycles inside host cells. As a general rule, these two modes of attack correspond to the two major classes of infectious disease-causing agents: bacteria and viruses.

Most bacteria are extracellular pathogens. As such, they live and thrive outside the host cells. For the bacteria the host is a sort of incubator and a source of nutrients, providing a warm and fertile environment for the bacteria to grow and multiply. But most bacteria are self-sufficient in at least one regard. They contain within themselves all of the cellular machinery required to multiply. Reproductively independent as they are, bacteria can in some but not all cases be less than benevolent to their human hosts. During the course of their life cycles, bacteria can manufacture and secrete biochemicals that are toxic to their hosts. Even though they remain outside host cells, bacteria can still cause disease via their toxins.

In contrast to the relatively independent bacteria, viruses spend only part of the time outside the host cells. When it comes time to multiply, they must enter the host cells. While viruses contain a complete set of their own genes, they carry little else inside their protein shells. Without any means to replicate their own genes or manufacture the proteins that encase those genes, viruses are compelled to enter a person's cells, making the infected person's cells an unwilling host to an all too often pernicious viral guest. The unwanted viral visitor co-opts host cellular machinery to carry out its own gene and protein synthesis. Viruses are therefore intracellular pathogens.

Since pathogens can exist either intracellularly or extracellularly, the immune system must be ready to respond in either context. Or, returning to the military analogy, the immune system must have at least two branches: one that is prepared to fight extracellular pathogens and another to fight intracellular ones—and indeed it does.

To fight extracellular pathogens, the body produces antibodies. These highly specific molecules are manufactured and secreted into the blood by a type of white blood cell called a B-cell. Once in the blood the antibodies can fight infectious agents that are swimming there. The



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