In presence of calcium ions, it binds to surfactant phospholipids and contributes to lower the surface tension at the air- liquid interface in the alveoli of the mammalian lung and is essential for normal respiration. Enhances the expression of MYO18A/SP-R210 on alveolar macrophages (By similarity). . (Microbial infection) Recognition of M.tuberculosis by dendritic cells may occur partially via this molecule (PubMed:17158455, PubMed:21203928). Can recognize, bind, and opsonize pathogens to enhance their elimination by alveolar macrophages (PubMed:21123169). {ECO:0000269|PubMed:17158455, ECO:0000269|PubMed:21123169, ECO:0000269|PubMed:21203928}. (Microbial infection) Binds M.pneumoniae CARDS toxin, serves as one receptor for this pathogen (PubMed:15845487, PubMed:25139904). When SFTPA1 is down-regulated by siRNA, less toxin binds to human cells and less vacuolization (a symptom of M.pneumoniae infection) is seen (PubMed:25139904). {ECO:0000269|PubMed:15845487, ECO:0000269|PubMed:25139904}. This is the function of SFTPA1 (surfactant protein A1, Ensembl gene identifier ENSG00000122852).