a-short-review-of-histology

Lymphocytes:

Lymphocytes are found as two (morphological) types: small and medium
Lymphocytes are a type of agranular leucocyte although some types of lymphocytes do contain very small cytoplasmic granules which are functionally important
Lymphocytes are cells which are central to the functioning of the immune system
Functionally, it is now known that there are many different types of lymphocyte (T, B, NK etc). These cannot be distinguished morphologically
However, they can routinely be classified according to the surface expression of specific proteins which can be identified using labelled antibodies (immunocytochemistry)
Small lymphocytes are about 8-9 microns in diameter ie about the size of a red blood cell
In a Romanovsky-type stained peripheral blood smear they appear as a round cell with a heterochromatic nucleus and little discernible cytoplasm
Medium lymphocytes are about 1O-12 microns in diameter and have a larger more euchromatic nucleus than small lymphocytes
They also have more cytoplasm
Medium lymphocytes are now more usually classified as activated lymphocytes because it is recognized that they arise from small lymphocytes as part of an immune response
Lymphocytes represent about 20-40% of white blood cells in normal circulating blood
The time these cells spend in the blood, and in tissue spaces, is highly variable largely depending on sub-type and function

Lymphocytes - 2:

In this image of a blood smear a small lymphocyte is labelled C (A = neutrophil, B = basophil)
The nucleus of the small lymphocyte appears to occupy the whole cell but if you look closely at the top edge of the cell you can see a small amount of cytoplasm
The small lymphocyte has a similar diameter to surrounding red blood cells but has a smaller diameter than the neutrophil and basophil also shown on this image

Lymphocytes - 3:

In this electron micrograph a medium (activated) lymphocyte is shown
By comparison with a small lymphocyte, the medium lymphocyte is larger, has a larger more euchromatic nucleus and a fair amount of cytoplasm
In this example, the lymphocyte appears to be passing between endothelial cells of the thin walled blood vessels (BV) into (probably?) tissue space (TS)