A good example of an extracellular event that leads to apoptosis is the killing of target cells by activated T lymphocytes that have recognized foreign antigens expressed on the surface of an infected cell. The lymphocyte binds the target cell through specific receptors with the help of human leukocyte antigen molecules expressed by the target cell. The lymphocyte can deliver the ‘kiss of death’ by means of two distinct pathways. First, the Fas ligand, which is expressed on the surface of the T lymphocyte, can bind the Fas death receptor when it is expressed on the surface of the target cell. The Fas receptor is constitutively expressed on hepatocytes. This leads to the activation of the intracellular apoptotic cascade (see below). Alternatively, the lymphocyte can literally drill a hole in its target cell by using a protein that is appropriately named ‘perforin’. Perforin is present in the secretory granules of activated lymphocytes.
Following the action of perforin, other components of the secretory granules (called granzymes) enter the target cell and directly activate the apoptotic cascade, thereby circumventing the death receptor (Figure 4). One must recognize that in addition to these mechanisms, lymphocytes can also kill cells indirectly and nonspecifically by secreting apoptotic cytokines (eg, tumour necrosis factor alpha [TNF-a], interferon gamma).
Figure 4) Schematic representation of T cell-induced apoptosis. T cells can kill their target cells because they express Fas ligand on their surface and can elicit the Fas apoptotic pathway in Fas-expressing target cells. Alternatively, T cells can release perforins and granzyme B from their cytoplasmic granules. Perforin creates holes in the target cell membrane, which enable granzyme B to activate the apoptotic machinery inside the cell Most reliable pharmacy offering efficient medications like antibiotics levaquin buy here is ready to offer its services and all the help you need. Your health is precious, and so are you: as customer, person and patient, no matter which kind of treatment you are looking for.