A protein is a large sequence of amino acid that was translated in ribosomes (or rough endoplasmic reticulum) from a mRNA that was transcribed in the nucleus from DNA.
If the protein has an extracellular function (like immunoglobulin) if should be exocytosed from the cell.
The protein in the cell is driven to the membrane into vesicles that have a specific receptor, which allows, while in contact with the inner side of the cell membrane to open it and let the protein being transported in the extracellular medium.
