( ( The spleen ) ( ( ( ( ( acts ( as ( a filter ) ) ) ( against ( ( foreign organisms ) ( that ( ( ( ( infect ( the bloodstream ) ) , ) and ) ( also ( ( filters out ) ( ( old ( red ( blood cells ) ) ) ( from ( the bloodstream ) ) ) ) ) ) ) ) ) ) and ) ( decomposes them ) ) . ) )
(ROOT (S (NP (DT The) (NN spleen)) (VP (VP (VBZ acts) (PP (IN as) (NP (DT a) (NN filter))) (PP (IN against) (NP (NP (JJ foreign) (NNS organisms)) (SBAR (WHNP (WDT that)) (S (VP (VP (VBZ infect) (NP (DT the) (NN bloodstream))) (, ,) (CC and) (VP (ADVP (RB also)) (VBZ filters) (PRT (RP out)) (NP (NP (JJ old) (JJ red) (NN blood) (NNS cells)) (PP (IN from) (NP (DT the) (NN bloodstream))))))))))) (CC and) (VP (VBZ decomposes) (NP (PRP them)))) (. .)))
The spleen acts as a filter against foreign organisms that infect the bloodstream, and also filters out old red blood cells from the bloodstream and decomposes them.
(ROOT (S (NP (NP (DT The) (JJ main) (NN function)) (PP (IN of) (NP (DT the) (NN spleen)))) (VP (VBZ is) (S (VP (TO to) (VP (VP (VB filter) (NP (DT the) (NN blood))) (CC and) (VP (VB remove) (NP (JJ unwanted) (JJ red) (NN blood) (NNS cells))))))) (. .)))
The main function of the spleen is to filter the blood and remove unwanted red blood cells.