Saliva contains a lot of protein. However, it also contains cells of normal flora and cell debris. You can extract the protein using centrifugation to separate cells and protein. To purify the extracted protein, you can use PAGE , filtration, and other methods. It depends on the level of purity you need.

Thank you Sir. I need to extract crude protein from saliva and then want to perform ELISA for the target protein

I agree with explanation given by Zulham Yamamoto

to remove host cell, bacteria and food debris. the best and simple method is to centrifugate for 20 min at 14,000 x g. refrigerated centrifuge. the supernatant will be basically salivary proteins from host and microorganism. 

it is difficult to remove all salivary proteins by using centrifugation.

Sure. what type of small molecules?

Anindita Sen i suggest using 10kDa spin column. They are effective in removing those large proteins from saliva. Expensive but efficient.