primary TB
Primary TB is the body's very first encounter with the tubercle bacillus — the infection that develops soon after someone breathes in the bacteria for the first time. It is the opening chapter of TB, before the immune system has any prior experience of the organism to draw on.
When the inhaled bacteria land in the lungs, they multiply in a small area, usually in the lower or middle zones where inhaled air is delivered most freely. This initial lesion, together with the swollen lymph nodes that drain it, forms a characteristic pair seen on imaging. In most healthy people the immune response then contains the infection, often leaving only a small scar or calcified node.
Primary TB is frequently mild or silent and is especially common in children. However, in young infants and people with weak immunity the first infection can progress directly to serious disease, including spread through the bloodstream. The contained, scarred-down result is what may later reactivate into post-primary TB if immunity wanes.
A young child exposed to a relative with TB develops a small mid-zone lung shadow with enlarged hilar lymph nodes on X-ray — the classic picture of primary TB.
Lung lesion plus draining node — the hallmark of primary infection.
The peripheral lung lesion of primary infection is the Ghon focus; together with its draining lymph node it forms the “Ghon complex.”