The Neutrophil Response and Granulocyte Maturation

The movement between these pools is dynamic. For example, during certain physiological responses, neutrophils may "flush" from the marginating pool into the circulating pool, causing a sudden increase in the number of cells detected in a standard blood test.

The Neutrophil Response to Inflammation

Inflammation triggers chemical mediators in the tissues that demand more neutrophils. The body responds in two primary ways: by increasing the movement of neutrophils into the tissues (vasodilation) and by increasing the rate of production in the bone marrow.

Terminology of the Response

To describe these changes, clinicians use specific terms regarding the quantity and maturity of the cells:

• Neutrophilia: An increase in the number of mature neutrophils.
• Neutropenia: A decrease in the number of mature neutrophils.
• Left Shift: An increase in immature (band) neutrophils.

The relationship between mature and immature cells allows us to categorize the severity of the inflammatory response:

Clinical Note: Toxic Neutrophils During acute inflammation, the shortened production time in the bone marrow can result in "toxic neutrophils". These cells exhibit morphological changes such as heavier granulation, vacuoles, or Dohle bodies in their cytoplasm. While these changes look alarming on a blood smear, they do not necessarily impair the cell's function.

Patterns of Inflammation

The timing and intensity of inflammation dictate the blood profile:

1. Acute Inflammation: (e.g., bacterial infection, early wound) Characterized by neutrophilia, a regenerative left shift, and toxic neutrophils.
2. Chronic Inflammation: (e.g., abscesses, endocarditis) Occurring for $>7$ days, the marrow has met the demand. You may see neutrophilia and perhaps a mild left shift, but toxic neutrophils are usually absent.
3. Severe/Overwhelming Inflammation: (e.g., sepsis, necrosis) The tissue demand is so high it exceeds the marrow's release rate, leading to neutropenia and a degenerative left shift.

Non-Inflammatory Responses: Excitement vs. Stress

It is critical to distinguish between inflammation and physiological responses to stimuli, as the latter do not involve a left shift.

The Excitement Response

Often seen in cats, horses, and young dogs, this is a "fight or flight" reaction caused by epinephrine release. The epinephrine increases blood flow to the microcirculation, flushing neutrophils from the marginating pool into the circulating pool.

• Key Findings: Neutrophilia, lymphocytosis (increased lymphocytes), and no left shift.

The Stress Response

This is caused by the release of cortisol due to systemic illness, pain, or metabolic disturbances (e.g., diabetic ketoacidosis). Cortisol decreases the "stickiness" of neutrophils in the marginating pool, allowing them to enter the circulation.

• Key Findings: Neutrophilia, lymphopenia (decreased lymphocytes), monocytosis, and potentially hypersegmented neutrophils (due to longer retention in circulation).

Stem Cell Injury and Pancytopenia

While neutropenia is often caused by the consumption of cells in tissues, it can also be caused by a failure of production in the bone marrow.

Stem cell injuries can be temporary or permanent and impact all blood cell lines produced by the marrow. When there is a reduction in all cell lines (erythrocytes, leukocytes, and thrombocytes), the condition is termed pancytopenia. Because neutrophils are the quickest to be replenished in the bloodstream, they are often the first cell line to show a decrease when the stem cell system is compromised.