An animal "knows" it is infected when infected cells die but are not
successfully scavenged before their internal contents leak out. Some of
these contents are captured by mobile antigen presenting cells, such as
dendritic cells, which travel to lymph nodes to initiate responses among
T cells specific for the new (presumably viral) antigens. Low grade
infections that do not cause massive cell death may not elicit immune
responses because no cellular leakage occurs prior to successful
scavenging by, say, macrophages.
Ephraim Fuchs
