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many diseases.
There are several types of leukocytes that differ in structure and function. Their
life expectancy varies from several days to several decades.
Leukocytes are continuously formed in the hematopoietic organs – the red bone
marrow, thymus, spleen and lymph nodes. They are destroyed in the spleen. White
blood cells constantly circulate in the blood and lymph.
Leukocytes play an important role in immune reactions – protecting the body
from foreign substances, cells and tissues.
IMMUNITY
Immunity is the ability of the body to protect itself from foreign bodies and
substances with the help of phagocytosis and the production of antibodies.
The mechanisms of immunity protect the body from infectious agents (bacteria
and viruses), free it from dead, as well as regenerated cells and, therefore, become alien.
In addition to leukocytes, the immune system includes bone marrow, thymus
(thymus gland), lymph nodes.
PHAGOCYTOSIS
Leukocytes are able to move actively. Some of their species can penetrate
through the walls of blood vessels into tissues, where they absorb and digest foreign
particles. In addition, they can recognize and destroy cancerous and old, dying cells.
This absorption process is called phagocytosis, and the leukocytes that carry it out are
phagocytes.
If a lot of foreign bodies have penetrated into the body, phagocytes, absorbing
them, greatly increase in size and are destroyed. At the same time, substances that cause
a local inflammatory reaction are released, which is accompanied by swelling, fever
and redness of the affected area.
Substances that cause an inflammatory reaction attract new leukocytes to the site

of the introduction of foreign bodies. Destroying foreign bodies and damaged cells,
white blood cells die in large quantities. The pus that forms in the tissues during
inflammation is an accumulation of dead leukocytes.
Antibody production
Protection of the body from foreign bodies is carried out not only with the help
of phagocytosis. The body also produces special substances of a protein nature –
antibodies that recognize and bind foreign agents (bacteria, protozoa, fungi) and
harmful substances (toxins) produced by them.
The harmful particles bound by antibodies cannot penetrate into human tissues
and become harmless.
The formation of antibodies occurs with the participation of lymphocytes – a
special type of leukocytes found not only in the blood, but also in the lymph.
Usually antibodies act against the causative agent of one disease. However, there
are known groups of diseases acting against pathogens. They increase the overall
resistance of the body.

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