( Finally ( , ( it ( ( has ( been ( proposed ( that ( ( the ( initial ( polio infection ) ) ) ( causes ( ( ( an ( autoimmune reaction ) ) , ) ( ( in which ) ( ( ( the ( body 's ) ) ( immune system ) ) ( ( attacks ( normal cells ) ) ( as ( if ( they ( were ( foreign substances ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB Finally)) (, ,) (NP (PRP it)) (VP (VBZ has) (VP (VBN been) (VP (VBN proposed) (SBAR (IN that) (S (NP (DT the) (JJ initial) (NNP polio) (NN infection)) (VP (VBZ causes) (NP (NP (DT an) (JJ autoimmune) (NN reaction)) (, ,) (SBAR (WHPP (IN in) (WHNP (WDT which))) (S (NP (NP (DT the) (NN body) (POS 's)) (JJ immune) (NN system)) (VP (VBZ attacks) (NP (JJ normal) (NNS cells)) (SBAR (RB as) (IN if) (S (NP (PRP they)) (VP (VBD were) (NP (JJ foreign) (NNS substances))))))))))))))) (. .)))
Finally, it has been proposed that the initial polio infection causes an autoimmune reaction, in which the body's immune system attacks normal cells as if they were foreign substances.
(ROOT (S (NP (DT The) (JJ immune) (NN system)) (ADVP (RB typically)) (VP (VP (VBZ recognizes)) (, ,) (VP (VBZ attacks)) (, ,) (CC and) (VP (VBZ destroys) (NP (NP (JJ foreign) (NNS cells) (CC or) (NNS substances)) (SBAR (WHNP (WDT that)) (S (VP (MD may) (VP (VB cause) (NP (NN disease))))))))) (. .)))
The immune system typically recognizes, attacks, and destroys foreign cells or substances that may cause disease.