Cells of the Innate Immune Response

The two essential cells involved in the innate immune responses are phagocytes and natural killer, or NK cells.

Phagocytes — microphages and macrophages — are involved in phagocytosis, a multi-step process to engulf cellular debris and foreign particles like bacteria.

Microphages comprise the abundantly circulating neutrophils and less abundant eosinophils.

In contrast, the macrophages are large active phagocytes, primarily derived from monocytes in the blood. They can be broadly classified as wandering and fixed macrophages.

During an infection, the neutrophils are the first responders, followed by the wandering macrophages.

The fixed macrophages are permanently located within specific tissues like the Kupffer cells in the liver.

NK cells are large granular lymphocytes comprising about 5–10% of lymphocytes in the blood and lymph.

Unlike the B and T lymphocytes, NK cells lack specificity and have a wide range of targets.

They identify infected body cells or cancerous cells by the presence of viral or tumor antigens that are absent on normal body cells.

NK cells directly bind and induce apoptosis in the infected target cell without engulfing the pathogens.