Plasma cells . Plasma cells . are small so that they slip between endothelial cells of capillaries to fight infection in the surrounding tissues have a great deal of rough endoplasmic reticulum to dispose of ingested pathogens have a great deal of rough endoplasmic reticulum reflecting the fact that they secrete a tremendous amount of protein (antibody) are large so that they can envelope their prey by phagocytosis

Plasma cells are also known as Plasma B cells.
They are one of the types of white blood cells which develop inside the bone marrow.
Inside the bone marrow are present the multipotent hematopoetic stem cells.
These cells differentiate to form the myeloid stem cells and the lymphoid stem cells.
The lymphoid stem cells develop into the lymphoblasts which further differentiate to form the B lymphocytes, T lymphocytes and Natural Killer cells respectively.
The B lymphocytes form the plasma cells.
These plasma cells produce antibodies which target foreign materials or organisms and tag them to be recognized and destroyed by other cells of the immune system like macrophages, T cells, Natural Killer cells, etc.
The antibodies are composed of proteins.
Hence, in order to produce huge amounts of antibodies (proteins) the B cells have huge quantity of rough endoplasmic reticulum (RER).
This is because secretory proteins or proteins present in the plasma membrane are synthesized by the ribosomes attached to the endoplasmic reticulum (ER), that is, the RER.