Being a gateway for molecules or ions
Because it’s right that proteins help the immune system but thus is not related with plasma membrane
Making the membrane more firm is the function of phospholipids
Making proteins it’s not right , the proteins are made from DNA —>mRNA——->peptide—->proteins
So being a gateway for molecules or ions is the right answer because in the plasma membrane of the cell we can find channels that allow the entrance of molecules for example aquaporins which allow entrance of H2O to the cell and they’re formed of proteins