text: SP-B is one of four proteins commonly found in surfactant, the other three being surfactant protein A (SP-A), surfactant protein C (SP-C), and surfactant protein D (SP-D). These four are highly interconnected in their functions in surfactant. For example, though the mechanism is not yet understood, SP-B functions in the post-translational modification of SP-C, and mature SP-C is not formed without SP-B.
  SP-C assists in the functions of SP-B, and is most similar to SP-B of the three other surfactant proteins. It is smaller, only 35 amino acids long, and is found embedded in lipid structures much like SP-B.
  SP-A and SP-D, known together as collectins, are more distinct from SP-B than SP-C. They are hydrophilic, so they are found in the solution, and function in immune response instead of lipid arrangement and surface tension reduction.  SP-A is actually a name for two very similar proteins, SP-A1 and SP-A2.
  Along with SP-A, B, C, and D, blood plasma proteins are found in very small quantities in surfactant as well.