Caption: Human T lymphocytes (pre-T cells) and activated platelet. Lymphocytes are involved in the specific immune response and are composed mainly of precursor T cells and B cells (pre-T cells and B cells). Pre-T cells and B cells are subsets of lymphocytes that originate in the red bone marrow from hematopoietic stem cells. Pre-T cells (also known as T lymphocytes) circulate in the blood before migrating to the thymus where they develop into specialized cells (helper T cells and killer T cells) that are able to identify antigens and infected tissue cells. Platelets are cell fragments in the blood that play an essential role in blood clotting and wound repair. Platelets can also activate certain immune responses. Platelets are formed in the red bone marrow, lungs, and spleen by fragmentation of very large cells known as megakaryocytes. Platelets in the blood are small oval disks and are termed nonactivated platelets or thrombocytes. Platelets serve as the body's first line of defense to prevent excessive blood loss. When an injury such as a cut is sustained, platelets (now known as activated platelets) change their shape, become sticky and build up on a vessel wall to form a plug. They are also involved in the secretion of a chemical platelet factor which helps produce threads of fibrin. Bloods cells collect in the entangled fibrin and platelet mass forming a blood clot at the site.
Magnification*: x2,200
Type: SEM
Copyright 2008 Dennis Kunkel Microscopy, Inc.
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