Surfactant protein structural shapes, illustration. Surfactant proteins are one of the components of pulmonary surfactant, the lipoprotein complex that is an essential component of a healthily functioning lung. Surfactant proteins also play a role in the immune system. This surfactant protein takes the form of a monomeric unit, three of which have formed a trimer at left. Six of these trimers have joined together to form what is called an octadecamer. The height of the octadecamer is shown as 20 nanometres. An example of a surfactant protein that organises in this way is surfactant protein A.