ial ▁type ▁II ▁cells ▁in ▁the ▁al ve olar . ▁This ▁mixture ▁is ▁made ▁and ▁pack aged ▁into ▁l ys os om ally - ▁derived ▁structures ▁called ▁l am ell ar ▁bodies . ▁Lam ell ar ▁bodies ▁are ▁then ▁secret ed ▁into ▁the ▁liquid - air ▁inter phase ▁surface ▁of ▁al ve olar ▁through ▁memb rane ▁fusion ▁initi ated ▁by ▁infl ux ▁of ▁Ca 2 + . ▁Re leased ▁pul mon ary ▁sur fact ant ▁acts ▁as ▁a ▁protect ive ▁layer ▁to ▁prevent ▁al ve olar ▁from ▁col laps ing ▁due ▁to ▁surface ▁t ension . ▁Furthermore , ▁sur fact ants ▁also ▁contains ▁some ▁inn ate ▁imm une ▁components ▁to ▁defend ▁against ▁pul mon ary ▁in fe ctions . ▁Sur fact ant ▁is ▁class ified ▁into ▁two ▁types ▁of ▁prote ins , ▁hydro ph il ic ▁prote ins ▁that ▁are ▁responsible ▁for ▁inn ate ▁imm une ▁system , ▁and ▁hydro ph ob ic ▁prote ins ▁that ▁carry ▁out ▁physical ▁functions ▁of ▁pul mon ary ▁sur fact ant . ▁Sur fact ant ▁met abol ism ▁d ys function ▁involves ▁mut ations ▁or ▁mal functions ▁of ▁those ▁hydro ph ob ic ▁prote ins ▁that ▁lead ▁to ▁in effect ive ▁sur fact ant ▁layer ▁to ▁protect ▁al ve ol us ▁integrity . ▁SP - B ▁and ▁SP - C ▁are ▁the ▁two ▁hydro ph ob ic ▁sur fact ant ▁prote ins ▁that ▁particip ate ▁in ▁its ▁physical ▁functions ; ▁these ▁prote ins ▁are ▁encoded ▁by ▁SF TP B ▁and ▁SF TP C ▁gen es ▁on ▁chrom