Key leukocytes of the innate immune system

If pathogens breach the body’s protective physical, chemical or microbiota barrier then the innate immune system responds using various white blood cells (leukocytes).The innate immune system responds non-specifically to any invading pathogen and can be considered the body’s second line of defense. There is also a third line of defense: The adaptive immune response . The adaptive immune response can be explored further here.

Most leukocytes of the innate immune system are involved in the process of inflammation which is explored in more detail here. Several leukocytes perform phagocytosis , which is described in more detail below. The innate immune system also employs some important chemical defenses using interferons and complement proteins which are outlined here.

Click on the hotspots in the image below to learn more about the leukocytes of the innate immune system.

Phagocytosis

Phagocytosis is the process in which innate immune cells engulf other cells or particles. Neutrophils, macrophages and dendritic cells are the key cells that perform phagocytosis.

Phagocytosis is important because it can achieve three goals:

  • Destroy pathogens by breaking them down into waste.
  • Access parts of pathogens to then present antigen information to other immune cells.
  • Clean up waste from damaged cells and destroyed pathogens.

The process of phagocytosis is initiated by chemical signals that something in the environment has changed. These can be chemicals released from pathogens, damaged cells, mast cells, or other leukocytes. The phagocyte then moves towards the signals and checks for foreign antigens. When it finds a foreign antigen it signals other nearby cells and commences phagocytosis, the process of engulfing and destroying the pathogen .

Note the key steps of phagocytosis in the following image. Whilst this example shows a bacterium, the steps can also be applied to most small pathogens.

Diagram of the key steps in phagocytosis. Shows a macrophage, a large cell with multiple pseudopodia. 1) a rod-shaped bacterium is engulfed by a pseudopodia and taken into the macrophage’s cytoplasm encased in a vacuole. 2) Lysosomes, multiple small round vesicles, fuse with the vacuole and digest the bacterium. 3) Antigens from the digested bacterium are combined with an MHC II receptor, and the complex moved to be presented on the surface of the macrophage

Source: OpenStax

Lysosymes are digestive enzymes produced internally by phagocytes and stored in special vacuoles called lysosomes. When lysosomes fuse with a vacuole containing a pathogen they form a vacuole called a phagosome, and the lysosymes start to break down the pathogen. This separates the antigens of the pathogen, making them available to be placed on MHC proteins and transported to the surface of the cell. The antigens are then displayed to specialised cells to initiate an adaptive immune system response. The rest of the pathogen is broken down as waste.

Diagram of the formation of a phagolysosome. On the left is a neutrophil, a rounded cell with a multilobed nucleus. It stretches a pair of pseudopodia around some bacteria. On the right is a panel that zooms in on the base of the pseudopodia. It shows that the neutrophil has surface receptors that have bound to the bacteria, and the bending of the cell membrane to begin forming a phagosome. A lysosome is present on the left inside the cell, a round vesicle containing digestive enzymes. Arrows point from both of these to the next stage. Here the phagosome has combined with the lysosome to form a phagolysosome, containing digested parts of the bacteria and digestive enzymes.

Phagocytosis can be enhanced through opsonisation. This is the binding to the outer surface of pathogens by antibodies (through the adaptive immune system response) or by complement proteins (through the innate immune system response).

Diagram illustrating opsonisation. On the left a a rod-shaped bacterium with a flagella, that has 3 Y-shaped antibodies bound to its surface. Below that is another bacterium that has two round complement proteins bound to its surface. On the right is a macrophage, a large with a prominent nucleus, that is engulfing bacteria bound by antibodies and complement proteins.

By binding to the pathogens, antibodies and complement proteins can inhibit the actions and movement of pathogens as well as make them easier for phagocytes to find and engulf.

The next section describes how the innate immune system cells recognise antigens to be able to distinguish normal healthy cells of the body compared to pathogens, virus-infected cells and cancerous cells.