Introduction

        An Overview of Infection and Immunity A Game of Chess Played by Masters Initiating an Infection

        Successful Infections Must Modulate or Bypass Host Defenses Skin Respiratory Tract Alimentary Tract Eyes Urogenital Tract Placenta

        Viral Tropism Accessibility of Viral Receptors Other Host-Virus Interactions That Regulate the Infectious Cycle

        Spread throughout the Host Hematogenous Spread Neural Spread

        Organ Invasion Entry into Organs with Sinusoids Entry into Organs That Lack Sinusoids Organs with Dense Basement Membranes Skin

        Shedding of Virus Particles Respiratory Secretions Saliva Feces Blood Urine Semen Milk Skin Lesions Tears

        Perspectives

        References

        Study Questions

      LINKS FOR CHAPTER 2

This earth of majesty, this seat of Mars This other Eden, demi-paradise This fortress built by Nature for herself Against infection and the hand of war.

      WILLIAM SHAKESPEARE, RICHARD II (ACT 2, SCENE 1)

Introduction

      Microbes are everywhere. They are on our hands, in our food, on the lips of those we kiss, on the ground and in the oceans, filling the air we breathe. For young children who play in dirt, scrape their knees, and pick their noses, interactions with microbes are even more frequent and diverse. As we begin a series of chapters dedicated to immune responses and viral diseases, perhaps the right question to ask is not “What makes us sick?” but rather, “How can we possibly manage to stay healthy?”

      If students of immunology are asked to list components of the host response to infection, typical responses will include mention of antibodies, cytotoxic T lymphocytes, and interferons. These answers are not incorrect per se , but to focus only on attributes of the immune system misses the bigger picture: by the time a virus or other pathogen has been engulfed by a phagocyte or induced a T cell response, it has already successfully bypassed an impressive fortress of defenses. These defenses, such as skin, mucus, and stomach acid, might seem much more primitive than the elegantly coordinated innate and adaptive immune responses. Nevertheless, they block the overwhelming majority of infections.

However, such sentries and barriers are imperfect, despite millions of years of evolution in the presence of microbes. When viruses breach these barriers, infections of host cells and attendant disease can occur. The genomes of successful viruses encode proteins that modify, redirect, or block these, as well as other, defenses. For every host defense, there will be a viral offense. It is remarkable that the genome of every known virus on the planet, no matter its size, encodes countermeasures to modulate the defenses