What are Platelets? Structure, How do Platelets work against pathogens?

What are Platelets?

What are Platelets? Platelets, or thrombocytes, are nucleated cells that come from the megakaryocyte cells in the bone marrow and are not only a key player in maintaining hemostasis but are also involved in the development of non-hemostatic immune functions.

  • Platelets are the second most abundant cells in the bloodstream with a concentration between 150 and 400 × 109 / l.
  • The involvement of platelets in innate immunity is the result of their ability to release a group of inflammatory mediators as a result of activation.
  • In addition, platelets can also act as effector cells by interacting with pathogens such as bacteria, viruses, fungi, and protozoa.
  • Some platelets secrete effector molecules that act as chemoattraction for macrophages, lymphocytes, and other immune cells.
  • The most important function of platelets is to initiate blood clotting and therefore they usually remain dynamic.
  • However, some platelets remain in an inactive state and are only activated when the blood vessels are damaged.
  • Platelets also influence adaptive immune responses as these express a wide range of immune receptors that can interact with immune cells acting as vascular endothelium.
  • Platelets are nucleated cells and therefore do not have a nucleus, but they are rich in nucleic acids such as mitochondrial DNA and RNA.
  • Platelets also share other characteristics with immune cells, such as the presence of granules in the cytoplasm which, when activated by exocytosis, release various mediators.

Structure of Platelets

Platelets are nucleated cells 1 to 3 µm in diameter but can increase in size up to 6 µm after activation.

Platelets are often viewed as cell fragments rather than actual cells because they lack many of the components of a normal cell.

The outermost region of the cell contains a membrane and its intussusception, which leads to the formation of an open channel system.

Under the membrane is a cytoplasm, which consists of a pronounced network of microfilaments that form an extensive cytoplasmic filament system.

Although platelets are anuclear, they contain cellular organelles such as RNA, ribosomes, mitochondria, and granules, which are essential for the cell to function.

Granules found in platelets are of three different types; α-granules, dense granules, and lysosomes.

α-granules are the most abundant and largest granules found in blood platelets. These contain platelet factors, which are responsible for hemostasis.

The dense granulate is the smallest granulate that appears as a dense body under an electron microscope. These contain ADH, serotonin, and high levels of calcium.

Lysosomes are also present in the platelet cytoplasm, which contain hydrolytic enzymes such as acid phosphatase and arylsulfatase.

How do Platelets work against pathogens? 

  • All platelets must undergo several changes in response to various stimuli. The activation of platelets causes different morphological changes in the cell.
  • The cell loses its round, spherical shape and, together with pseudopodia, has an irregularly shaped appearance.
  • The storage granules found in the cytoplasm fuse with the cell membrane, triggering the release of a set of cytokines and the expression of surface molecules.
  • In addition, the affinity of the adhesion molecules such as GPIb-V-X complexes is upregulated during activation, which leads to increased platelet adhesion and aggregation.
  • Surface receptors found in platelets include receptors such as TLRs, which are essential for activating various immune cells.
  • It has also been studied that TLR4, which induces platelet activation, is the result of increased adhesion to PMN cells. This further enhances the activation of neutrophils and the formation of extracellular neutrophil traps, thereby preventing the spread of bacteria.
  • Platelets also express CD154 receptors, which directly stimulate endothelial cells to induce inflammation on the wall of the blood vessel.
  • The other set of receptors found in platelets is Fc? RIA, glycoprotein VI, and C-type lectin receptor; However, the presence of these receptors depends on the individual characteristics of the cells.
  • The binding of these receptors to the fibrinogen is responsible for the activation of blood platelets and the release of cytoplasmic granules.
  • The activation of platelets is associated with the release of several molecules into the bloodstream. These molecules can trigger inflammation or other immune responses in the body.

Functions of Platelets 

The following are some of the functions of platelets:

  • The most important function of platelets is to maintain hemostasis by aggregating with other platelets and initiating a clotting cascade that prevents significant blood loss.
  • Platelets are among the first cells to recognize damage to endothelial tissue and the presence of microbial pathogens.
  • Various platelets also express pattern recognition receptors such as TLLR, which recognize pathogen-associated molecular patterns and can induce an effective immune response.
  • During activation, platelets release several mediators that help recruit immune cells as well as activate cells with adaptive immunity.
  • The degranulation of platelets leads to the release of compounds that act as chemoattractants for other immune cells such as macrophages, neutrophils, and dendritic cells.
  • In organs such as the lungs, skin, and kidneys, platelets affect leukocyte recruitment during inflammation.