Two types of Pneumoeystis carinii infections must be distinguished:
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Interstitial plasmaeell pneumonia, which occurs only in premature or in marasmic infants between the 10th and 24th week of life. It is most frequently associated with severe diarrhoea, atrophy of the bowel mucosa and IgG levels of below 200 mg~ . Pneumocystis carinii organisms spread rapidly throughout the alveoli of all pulmonary lobes in these debilitated infants. The capsular antigen of the Pneumocystis carinii cysts elicits a massive plasmacell infiltration of the alveolar septa, which is accompanied by immuneglobulin production of IgM type. This reaction occurs within 24 to 48 hrs and leads frequently to death. ~orphometric counts on 65 cases showed that up to 78% of the entire lung space was formed by septa distended with plasmacells and proplasmacells. The organisms disappear from the alveoli within 2 to 3 days due to the hosts IgM antibody response. The cellular infiltrate persists and may in itself cause suffocation. Complete clearing of the interstitial infiltrate takes 6--8 weeks. This sudden massive infiltration of the entire lung does not occur in all cases. Pneumocystis carinii organisms occur frequently in the subpleural posterior alveoli, elieiting a minor focal interstitial plasmacell response, which remains clinically undetected.
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ttypoergie Pneumocystosis may occur at any age and is associated with congenital immunedeficiency diseases, diseases of the reticulo-endothelial system and immunosuppressive therapy leading to IgG deficiency. No host response to the organism occurs under these circumstances. Morphometric counts on 51 human lungs showed no interstitial inflammatory infiltrate. The septa may occasionally be distended by neoplastic infiltrates of reticulo-endothelial type as for instance in plasmaeell myeloma, lymphoma or leukaemia, which is the cause rather than the result of the Pneumoeystosis.