Immune System
Polymorphonuclear leukocytes (or PMNs)

can be attracted to the site of an infection following activation of complement
are a type of phagocyte that can engulf and kill bacteria
All of these
are derived from bone marrow stem cells

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Immune System
T cell receptors (or TCR) on CD4+ T cells

recognize peptides associated with MHC class II molecules
recognize peptides not associated with MHC molecules
are secreted out into the environment to bind antigens
recognize peptides associated with MHC class I molecules

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