1.
The second type of cells in the blood (after the erythrocyte) is the leukocyte (= white blood cell).

2.
Their main function is to protect us from infection caused by invading bacteria, viruses etc.

3.
In contrast to the erythrocytes (and the platelets), there are many types of leukocytes.

4.
It is probably the easiest to separate those blood cells into two major types:
a. myelocytes
b. lymphocytes

1.
The myelocyte is really a very big family of related white

2.
It starts (in the bone marrow) with the myeloblast which then differentiates into several types:
1) megakaryocytes
2) neutrophils
3) eosinophils
4) basophils
5) monocytes

3.
The megakaryocytes are the cells that make the thrombocytes (=platelets). These will be discussed in the next page: Link:

4.
The neutrophils, eosinophils and basophils are closely related to each other. They are called ‘polymorphonuclear’ because they contain multiple nuclei within the cell body. These multiple nuclei look ‘granular’ under the microscope and that is why they are also called ‘granulocytes’.

5.
The monocytes are much simpler and not granular. But, when they leave the bone marrow and approach an infection, they swell and become very big. They are then called ‘macrophage’.

6.
Remember: the granulocytes and monocytes are produced in the bone marrow. They stay there most of the time until they are needed somewhere in the body.

1.
The lymphocytes are not produced in the bone marrow but in various ‘lymphoid’ tissues throughout the body.

2.
Lymphoid tissues are located in the lymph nodes, in the spleen, thymus, tonsils and a few other specialized locations (Peyer’s patches in the gut for example).

3.
Most of these lymphocytes remain in their locations while a few travel around in the blood.

4.
As soon as they are needed, a lot of lymphocytes are quickly developed and surge to the location of the ‘crime scene’!

1.
The function of all white blood cells is to combat infections (= inflammation).

2.
They do this by using three processes:
a) chemotaxis
b) diapedesis
c) ameboid motion

3.
When a tissue, somewhere in the body, becomes inflamed, the inflammation produces a lot of chemicals. These are called ‘chemotactic’ signals.

4.
These chemicals or ‘signals’ can be toxins from the bacteria or viruses, products from the dying cells in the tissues, products from the clotting taking place there, etc. All these chemicals diffuse, from the infected area, through the interstitial fluid to nearby capillaries.

5.
In the blood, these chemicals quickly flow to the bone marrows and alarm the leukocytes into action!

6.
The bone marrow activates the leukocytes to flow into the blood and stream to the site of inflammation. When they arrive in the capillaries closest tot the inflammation, they will migrate out of the capillaries and move towards the site of inflammation, using diapedesis and amoeboid motion. (‘amoeboid’ = moving like an amoeba!)

1.
Once the leukocytes have arrived at the site of inflammation, what do they do?

2.
One major function is: phagocytosis. They eat whatever is the cause of the inflammation; the bacteria, or the viruses or whatever caused the inflammation.

3.
Once the bacteria, or whatever the offending agents were, are engulfed by the leukocytes, the lysosomes inside the leukocytes will dissemble these particles.

4.
These lysosomes are filled with proteolytic enzymes of various types to digest those bacteria. (proteolytic: proteo = protein and lysis = destroy).

5.
Unfortunately, not ALL bacteria or viruses can be killed and removed in this way.

6.
Some bacteria are very resistant to these lysosomes. Examples are the tuberculosis bacillus. This may therefore induce tuberculosis!