WHITE BLOOD CELLS Study Notes

The human body is continually exposed to a variety of infectious agents:
- Types of infectious agents:
- Bacteria
- Viruses
- Fungi
- Parasites
These agents occur normally and can be found in varying degrees in:
- Skin
- Mouth
- Respiratory Tract
- Gastrointestinal Tract (G.I.T)
- Urinary Tract
Exposure to highly infectious agents can lead to serious disease or death if they penetrate deeper tissues.
The body has a specialized system for combating infectious agents, consisting of:
- Blood leukocytes (white blood cells)
- Tissue cells derived from leukocytes

White blood cells work in two primary ways to prevent disease:
1. Destruction of Invading Agents: This is achieved through a process known as phagocytosis.
2. Formation of Antibodies: This encompasses the creation of sensitized lymphocytes, which may directly destroy or inactivate the invading pathogens.

Role: They act as a mobile unit of the body’s protective system.
Formation:
- Partially formed in the bone marrow
- Partially formed in lymph tissue
After formation, white blood cells are transported throughout the body to areas needing defense.
An adult human has approximately 7000 white blood cells per microliter of blood.
White blood cells exist in six primary forms:
- Eosinophils
- Granulocytes
- Basophils
- Monocytes (5.3%)
- Lymphocytes (30%)
- Plasma Cells
- Neutrophils (62%)
- Eosinophils (2.3%)
- Basophils (0.4%)

Function: Primarily attack and kill invading bacteria, viruses, and other harmful agents.
Neutrophils:
- These are matured cells that can directly target bacteria in the circulating blood.
Macrophages:
- Originate from blood monocytes, which are immature and have limited ability to fight infections before entering tissues.
- Upon entering tissues, macrophages swell, increasing their volume by up to fivefold.

Definition: A process where white blood cells squeeze through the pores of blood capillaries.
Process: A small portion of the white cell slides through a pore at a time.

This describes how white blood cells move within tissues via a method similar to their namesake (amoeba), which allows for cell movement in relation to surrounding areas.

Definition: The attraction of white blood cells towards invading agents, driven by chemical substances such as:
- Bacterial or viral toxins
- Degenerative by-products
- Reaction products from the complement complex.

Definition: The cellular ingestion of offensive agents. It must be selective to avoid the ingestion of normal cells.
Conditions Influencing Phagocytosis:
1. Natural structures in tissues typically have smooth surfaces that resist phagocytosis; a rough surface is more likely to be ingested.
2. Many natural substances in the body possess protective protein coats that repel phagocytic action.
3. The immune system can develop antibodies that adhere to infectious agents, making them more susceptible to phagocytosis.

Process:
- Upon approaching a particle to phagocytize, the neutrophil attaches to the particle and extends pseudopodia in multiple directions.
- The pseudopodia meet on the opposite side and fuse, creating an enclosed space around the particle. This space is called a phagocytic vesicle (phagosome).
- A single neutrophil can ingest between 3 to 20 bacteria before it becomes inactivated and dies.

Capability: Macrophages, which are derived from monocytes, can engulf up to 100 bacteria.
They can also ingest larger particles such as erythrocytes and, occasionally, malarial parasites.
After digesting particles, macrophages can excrete residual products and often survive long-term, functioning for many months.

Both neutrophils and macrophages contain numerous lysosomes filled with proteolytic enzymes for the digestion of bacteria and other foreign agents.
Interaction With Phagosome: The lysosomes from either neutrophils or macrophages fuse with the phagosome, releasing enzymes and bactericidal agents into the vesicle.
- This initiates digestion and inactivation immediately.
These immune cells possess bactericidal agents which can kill most bacteria, even if lysosomal enzymes fail to digest them.

Process: When neutrophils and macrophages engulf significant quantities of bacteria and necrotic tissue:
- The neutrophils and many macrophages ultimately die.
- Over several days, a cavity may form within inflamed tissues, filled with a mixture of necrotic tissue, dead neutrophils, dead macrophages, and tissue fluid.
Resulting Substance: This mixture is termed pus.

Function: Eosinophils are produced in large quantities during parasitic infections.
Challenge: Most parasites are too large to be phagocytized by eosinophils.
Mechanism: Eosinophils attach to parasites and release substances that can kill many of them.
Associated Condition: The increase of eosinophils in the bloodstream is known as eosinophilia, often occurring during parasitic infections or allergic reactions.

Comparison: Basophils share similarities with large tissue mast cells, which are located outside many capillaries.
Functions:
- Liberate heparin into the blood, which inhibits blood clotting.
- Release histamine, a crucial component in allergic reactions.

Definition: A clinical condition characterized by the production of very few white blood cells by the bone marrow.
Implications: This condition leaves the body unprotected against various bacteria and other harmful agents.