Classification and main functions
Depending on whether specific granules are present in the cytoplasm, all leukocytes are generally divided into two large groups. The first group is granulocytes, which include neutrophils, eosinophils, and basophils. The second group is agranulocytes, which lack such granulation; it includes lymphocytes and monocytes.
The primary purpose of these nucleated blood cells is the protective function. It is implemented in the body through several crucial mechanisms:
- Phagocytosis. This is the process of active capture and engulfment of various microorganisms, foreign particles, and breakdown products of the body's own tissues by cells.
- Immunity. Leukocytes are responsible for the full realization of both humoral and cellular immune responses.
- Regulation. Cells are capable of producing and inactivating a multitude of various biologically active substances.
Unique properties of leukocytes
The main distinguishing feature of white blood cells is their pronounced motility. This complex movement process is ensured by the presence of specialized contractile proteins inside the cells — actin and myosin.
A crucial property of leukocytes is diapedesis — the ability of cells to actively leave the bloodstream. They exit vessels, passing directly between endothelial cells. To successfully cross the basement membrane of the endothelial layer, leukocytes secrete special enzymes that temporarily disrupt its structure.
The directed movement of cells in tissues is called chemotaxis or chemokinesis. Such migration is strictly regulated by chemical substances called chemoattractants. They determine the direction of cell movement toward the focus of inflammation or injury.
Leukocytosis
Any change in the number of white blood cells is always calculated per unit volume of circulating blood. The condition in which the number of leukocytes increases is called leukocytosis. It is subdivided into several types depending on the underlying cause:
- Functional (physiological). Occurs in completely healthy individuals. Observed during pregnancy, after eating (so-called digestive leukocytosis), or as a result of prolonged physical work (myogenic leukocytosis).
- Reactive. Develops as a response to various pathologies. Most often accompanies inflammatory processes, as well as massive cell and tissue damage (e.g., in myocardial infarction, stroke, or severe soft tissue injuries).
- Pathological. Occurs in specific blood diseases — leukemias. Its main and most dangerous feature is a pronounced disruption of the normal functional activity of circulating cells.
Leukopenia
Leukopenia is a decrease in the number of leukocytes per unit volume of circulating blood.
According to its origin, it can be primary (i.e., congenital) or secondary. The secondary form arises as a consequence of negative external impacts on the body. The main causes of its development include radiation damage, severe poisoning, and the intake of certain medications.
The clinical significance of this condition requires special attention. Pronounced leukopenia inevitably leads to a sharp decline in the patient's immunity. In this condition, the humoral and cellular links of immune defense are disrupted, and the phagocytic reaction is significantly suppressed, leaving the body defenseless against infections.
Briefly: answers to topical questions
What specific functions do neutrophils perform?
Neutrophils act as the first line of defense, being the first to contact the infectious pathogen. Their specific functions include:
- Blocking the penetration of the agent into the body.
- Destruction of the pathogen via phagocytosis and exocytosis.
- Adhesion to the endothelium and exit from the bloodstream.
- Chemotaxis (directed movement).
- Degranulation and engulfment.
- Secretion of $O_2$-dependent and $O_2$-independent factors.
After performing the function of digesting the pathogen, polymorphonuclear leukocytes die.
What consecutive stages does the process of phagocytosis consist of?
The process of phagocytosis can be divided into 4 or 8 consecutive stages. The basic division includes approach, recognition and adhesion, engulfment, and destruction. According to modern understanding, there are 8 stages:
- Chemotaxis — directed movement of the phagocyte toward the target.
- Adhesion — establishing contact and sticking.
- Membrane activation — preparation for engulfment.
- Immersion — wrapping around the object.
- Phagosome formation — closure of the membrane and internalization.
- Phagolysosome fusion — formation of the phagolysosome.
- Killing and digestion — breakdown of the target.
- Release of degradation products — removal of residues.
To remember the types of granulocytes, use the mnemonic: Basophils, Eosinophils, Neutrophils.
Class and exam questions
What is leukocyte diapedesis?
This is the unique ability of leukocytes to exit blood vessels into surrounding tissues. The process is carried out by penetrating between endothelial cells and destroying its basement membrane using secreted enzymes.
How does physiological leukocytosis differ from reactive leukocytosis?
Physiological (functional) leukocytosis occurs in healthy individuals in response to pregnancy, food intake, or physical labor. Reactive leukocytosis is always associated with pathology — inflammation or tissue damage in infarctions and trauma.
Why are patients with severe leukopenia susceptible to infections?
Because a decrease in the number of leukocytes leads to a global impairment of immunity. The patient's phagocytic reaction is suppressed, and the functioning of cellular and humoral defense links is disrupted.
What else to study on this topic
- Mechanisms and stages of microorganism phagocytosis
- Types of chemoattractants and biochemical regulation of chemotaxis
- Differences between humoral and cellular immunity
- The role of contractile proteins (actin and myosin) in cell movement
- Characteristics of enzymes that degrade the basement membrane during diapedesis
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Leukocytes" and 4,128 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the "Blood" section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.