Antigen-presenting cells (APCs) are immune cells such as B cells, macrophages and dendritic cells that detect and engulf and kill foreign objects and microorganisms. Antigens against the human immune system will react will then be presented on the cell surface of the APCs on special protein structures. Mast cells, basophils and eosinophils can also become APCs under certain conditions. 

This process is essential for initiation of adaptive immunity. The latter develops once the human immune system encounters a foreign substance, which it will neutralise with specifically tailored antibodies. Generation of memory cells allows us to create a stronger and faster response to an antigen if it is reencountered a second time.