Mast Cells and Platelets
Mast cells are located predominantly in barrier tissues such as the skin and mucous membranes. Their main feature is the presence of large granules containing preformed histamine, leukotrienes, interleukins, and prostaglandin D2. In addition, they can synthesize protease enzymes (chymase and tryptase) de novo. Through this arsenal, mast cells regulate smooth muscle tone, stimulate mucus secretion, influence gastrointestinal motility, and play a key role in allergic reactions and hemostasis.
Platelets, beyond their classic function of arresting bleeding (hemostasis), actively participate in inflammation and tissue repair. Activated platelets can directly destroy bacterial cells and interact with leukocytes. They secrete a range of biologically active molecules: adhesion molecules (selectins), serotonin, thromboxane A2 (which induces vascular smooth muscle spasm), as well as chemokines and proinflammatory cytokines, including interleukin-1 beta.
Natural Killer Cells: Guardians of Genetic Integrity
NK cells (Natural Killer cells) are large granular lymphocytes derived from a lymphoid progenitor. Unlike other lymphocytes, they lack a T-cell receptor but express surface markers CD16, CD56, and CD94.
Their primary targets are cells infected with viruses or intracellular parasites, as well as tumor cells. The term "natural" indicates that they do not require prior stimulation to initiate an attack.
The decision to destroy a target is based on a "signal balance." An NK cell contacts target receptors (such as major histocompatibility complex class I molecules, MHC I) via its receptors, such as KIR. If the target cell is healthy and expresses a normal level of self MHC I, an inhibitory signal predominates, blocking the attack. If MHC I is altered or downregulated due to viral infection or tumorigenesis, the inhibitory signal drops sharply. The activating signal prevails, and the killer cell releases cytotoxic granules.
In addition to direct destruction, activated NK cells secrete cytokines, notably interferon-gamma, fulfilling an important regulatory function.
Mechanisms of Cellular Cytotoxicity
There are several pathways by which NK cells destroy target cells:
- Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC). The killer cell surface bears the CD16 receptor, which binds to the Fc region of IgG antibodies coating target cell antigens. The antibody acts as a bridge, triggering a destruction process combining apoptosis and necrosis.
- Secretory (Perforin-Granzyme) Lysis. The killer cell releases specialized proteins into the immunological synapse. Perforins insert into the target cell membrane and form pores. Granzymes enter through these pores, triggering a cascade of reactions leading to DNA fragmentation and cell lysis.
- Non-Secretory (Receptor-Mediated) Lysis. This occurs without granule exocytosis, solely through cell-to-cell contact. The killer cell expresses Fas ligand, which binds to CD95 (the Fas death receptor) on the target. This immediately delivers an apoptotic "suicide" signal to the target cell.
Dendritic Cells
Dendritic cells are the primary professional antigen-presenting cells. They derived their name from the Greek word dendron (tree) because their immature forms possess long, distinct cytoplasmic projections.
Immature dendritic cells continuously patrol tissues, migrating to sites of tumor growth, tissue damage, or pathogen entry. Upon encountering a foreign antigen, the cell engulfs it via endocytosis or phagocytosis. This uptake triggers maturation. A mature dendritic cell undergoes a dramatic morphological change: its processes flatten, surface area increases, and it transforms into a veiled cell.