The rough endoplasmic reticulum synthesizes proteins. Most of the body's cells, except for red blood cells and sperm cells, contain a rough endoplasmic reticulum, and the exact proteins it synthesizes depend upon the type of cell.
All eukaryotic organisms contain endoplasmic reticula in their cells. There are two types of reticula: the smooth endoplasmic reticulum and rough endoplasmic reticulum. Both of these form a network of membranes and tubes stretching from the cell membrane to the nuclear envelope. The rough endoplasmic reticulum gets its name because the abundance of ribosomes in its membranes give it a rough appearance. These ribosomes enable the rough endoplasmic reticulum to synthesize proteins and frequently bind and unbind from the reticulum, depending on whether protein synthesis is necessary. This ensures that a cell, and the body, does not flood with unnecessary proteins.
In white blood cells, or leukocytes, the rough endoplasmic reticulum is the source of antibodies for fighting disease. In pancreatic cells, it produces insulin. The rough endoplasmic reticulum also helps create lysosomal enzymes, which digest cellular refuse. All of the proteins that the rough endoplasmic reticulum creates are secretory proteins; these are proteins that eventually leave the cell for use elsewhere.
The smooth endoplasmic reticulum contains no ribosomes and manufactures lipids and proteins. This reticulum plays a vital role in muscle function and produces hormones in the brain.