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

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

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Immune System
Polymorphonuclear leukocytes (or PMNs)

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

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