Identification and Function of Blood Cells

Hemoglobin and Oxygen Transport

The functional capacity of an erythrocyte is driven by its chemical composition. Each individual cell contains approximately 280 million hemoglobin molecules. Hemoglobin acts as the binding site for oxygen molecules.

In a physiological context, it is important to distinguish between the states of these molecules:

• Oxyhemoglobin: Hemoglobin bound to oxygen.
• Deoxyhemoglobin: Hemoglobin that is not currently bound to oxygen.

Interestingly, in many physiological states, more than 90% of cells may exist in a deoxyhemoglobin state.

Leukocytes: The Body's Defense System

Leukocytes, or white blood cells, serve as the primary defense mechanism against foreign invaders, including viruses, parasites, toxins, and other unwanted particles. They protect the body through several methods: phagocytosis (engulfing particles), secreting destructive chemicals, or initiating adaptive immune responses.

A unique characteristic of leukocytes is diapedesis. Unlike erythrocytes, most leukocytes have the ability to squeeze through capillary walls to migrate out of the bloodstream and into surrounding connective tissues to reach the site of an infection.

Classification of Leukocytes

Leukocytes are classified into two main categories based on their appearance under Wright's stain:

1. Granular Leukocytes: These cells contain granules in their cytoplasm that stain very darkly with Wright's stain. Their nuclei often appear lobed. This group includes:
   • Neutrophils
   • Eosinophils
   • Basophils
2. Agranular Leukocytes: These cells do not have deeply stained granules in their cytoplasm.

Exam Tip: When identifying cells under a microscope, always look at the nucleus first. If the nucleus is highly lobed and the cytoplasm has dark granules, you are likely looking at a granular leukocyte.

Focus on Neutrophils

Neutrophils are the most abundant type of leukocyte, accounting for 60-70% of all white blood cells. High counts of neutrophils in a blood sample are typically associated with a bacterial infection.

Neutrophils employ several sophisticated tactics to fight infection:

• Phagocytosis: They engulf and digest bacteria.
• Enzymatic Destruction: They release powerful enzymes from lysosomes to destroy bacterial cell walls.
• NETs (Neutrophil Extracellular Traps): They can release extracellular DNA that forms web-like fibers to trap and destroy bacteria. Note that releasing a NET requires the neutrophil to undergo a form of programmed cell death.

As neutrophils age, their nuclei change shape. Younger neutrophils have rod-shaped nuclei and are called "bands," while older neutrophils have many lobes and are referred to as polymorphonuclear leukocytes or "polymorphs".

Platelets: The Clotting Agents

Finally, blood smears reveal platelets, also known as thrombocytes. While they are much smaller than erythrocytes and leukocytes, they play a vital role in hemostasis (the stopping of blood flow) through the formation of blood clots.

Summary Table of Blood Cell Components