Blood and lymph

Intracellular pathogen response: Virus

Recall that many pathogens may be evaded through the first-line “barrier defenses” in the respiratory mucosa, such as intact epithelium, mucus production, and the beating of cilia. However, viral particles in respiratory droplets have succeeded in infecting cells in A.P.’s respiratory tract. Some infected cells die, releasing products normally sequestered intracellularly and alerting the cells of the innate immune system. Among these, macrophages are prominent, as they clean up the debris from dead cells.

The macrophages are activated, releasing inflammatory mediators, such as interleukin-1 (IL-1). However, the virus continues to spread. Infected cells load some viral peptides into major histocompatibility complex (MHC) I molecules and display them on their surface, essentially sending out “red flags.” In addition, the infected cells may secrete mediators called interferons that induce a relative state of viral resistance in the neighboring cells most at risk for infection by the spreading virus (thus interfering with the viral life cycle). Eventually, cytotoxic T cells with receptors that recognize the viral antigens in the MHC I molecules displayed by infected cells arrive at the scene and kill off these compromised body cells. The infection is thus brought under control as the virus is eliminated.

Interferons and related signaling molecules, such as IL-1, have systemic effects causing fever, loss of appetite, weakness, and muscle aches. Additionally, local inflammation leads to sore throat and cough because of irritation of respiratory tract receptors.