Lymphocytes and Microscopy
Staining, Observations, Discussion
Lymphocytes are leukocytes that develop from the common lymphoid progenitor. Although they primarily reside in the lymph nodes, they increase in size and increasingly divide once activated and migrate to the infected tissue where they destroy the infecting pathogen.
The different types are differentiated by their respective cell surface receptors and specific function with regards to immunity.
- B cells (B lymphocytes)
- T cells (T lymphocytes)
- Natural killer cells
For a healthy person, lymphocytes make up
between 25 and 30 percent of the total leukocyte population, which translates
to between one and two trillion of these cells in a healthy individual.
from their ability to quickly respond to invading pathogens, some (particularly the B and T lymphocytes) have a unique characteristic
in that they can retain memory of antigens they may have previously
encountered. This makes it possible for these cells to immediately destroy such
antigen when they encounter them again. For this reason, they are also referred
to as memory cells.
To identify these cells in a blood smear,
Wright's stain can be used. This is an important stain that is often recommended
for differential staining of blood smears or bone marrow.
- Place a drop of blood onto
a clean glass slide
- Using another clean slide
or cover slip at an angle, spread the drop of blood (or bone marrow) to create
a thin film of blood
- Place the slide on a rack
and allow to air dry
- Once the slide is
completely dry, use a pipette to add 1.0ml of Wright's stain solution on the
thin film of blood and allow to stand or about 3 minutes
- Add a few drops of
distilled water or phosphate buffer (pH 6.8) and allow to stand for about 5
- Rinse the slide using water
- Allow the slide to dry and
wipe away any excess stain at the back of the slide
- View the slide under the
microscope starting with low power
When viewed under the microscope, lymphocytes
will appear dark purple with a deep bluish nucleus and a sky blue cytoplasm.
Like monocytes, lymphocytes are agranulocytes,
which simply means that they do not have granules. However, some of the larger cells (7-10um) may contain a small amount of scattered granules.
Ranging between 20 and 30 percent, they
are the second largest population of leukocytes, which makes it easier to
identify in a blood smear. While both the B and T cells are produced in the
bone marrow, T cells go on to mature in the Thymus.
B Cells, T Cells and Natural Killer Cells
As already mentioned, their primary function is to fight and destroy antigens. This task is achieved through two
major types of responses namely; humoral immunity and cell mediated immunity.
Whereas humoral immunity relies on the capacity to identify antigens to be
destroyed, cell mediated immunity involves the active destruction of both the
infected cells and cancerous cells.
In the presence of antigens, B cells are
activated, which results in the creation of antibodies specific to the antigen
in question. As such, they play an important role in humoral immunity. Given
that the antibodies remain in circulation, they are able to identify the
specific antigens in the future (memory cells).
On the other hand, T cells play an
important in cell mediated immunity. Here, cells move to the site of infection where
they attack and destroy the foreign particles.
There are different types of T
Cytotoxic T cells - Responsible for destroying
cancerous cells or infected cells.
T helper cells - Participate in the production
of antibodies in addition to the activation of given T cells.
Regulatory T cells - Regulatory T cells are
also known as suppressor cells and play an important role of suppressing given
responses by both B and T cells.
* Natural killer cells play an important role in
the rejection of tumors as well as infected cells. However, their response
tends to be non-specific compared to T and B cells.
See Also: Basophils, Neutrophils, Monocytes, Eosinophils
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