Ghon focus
A Ghon focus is the small spot in the lung where TB first takes hold after someone breathes in the bacterium. Think of it as the bacterium's landing site — a tiny patch of inflammation, typically near the edge of the lung in the well-ventilated lower or middle zones, that marks the point of first contact.
At this site the immune system mounts a walling-off reaction, surrounding the bacteria with a granuloma. Over time the focus often heals into a small fibrous or calcified scar, which can remain visible on a chest X-ray for life as a quiet reminder of a past primary infection that the body brought under control.
The Ghon focus rarely travels alone. Together with the enlarged lymph node that drains it toward the lung's central hilum, the pair forms the Ghon complex, the classic radiological signature of primary TB. In most healthy people this is the end of the story; only if immunity later fails can dormant bacteria within reactivate.
A routine chest X-ray in a healthy adult shows a tiny calcified nodule with a calcified hilar node — an old, healed Ghon complex from childhood primary TB.
A calcified scar marking a long-controlled primary infection.
A Ghon focus plus its draining hilar lymph node is the Ghon complex; when both have calcified it is sometimes called a Ranke complex.