Infection represents a failure of tissue homeostasis caused
by the presence of replicating foreign organisms.
Once microorganisms enter normally sterile tissue, they may
replicate rapidly within extracellular space or host cells. However, the
adaptive immune system cannot respond immediately. Antigen-specific lymphocytes
are initially present at extremely low frequency, meaning that several days are
required for recognition, proliferation and differentiation into functional
effector cells.
This delay creates a physiological constraint: pathogen
replication may outpace adaptive immune activation unless early containment
mechanisms limit microbial expansion and generate the signals required to
initiate antigen-specific responses.
The immune response therefore proceeds as a sequence of
linked events in which early innate mechanisms modify the local tissue
environment in ways that enable later adaptive immunity.
Each stage both responds to the current threat and prepares
the conditions required for the next phase of host defence.
Understanding how this occurs explains why immune responses
evolve over time and why different immune deficiencies produce distinct
clinical patterns of disease.
