Final answer:
Antigen recognition occurs when T cells identify antigens presented in combination with major histocompatibility complex molecules on the surface of antigen-presenting cells. B cells also recognize antigens directly but then process and present them with MHC class II molecules slightly differently than T cells.
Step-by-step explanation:
Antigen recognition takes place on the surface of antigen-presenting cells (APCs). Immune system T cells recognize antigens not in their original form but when they are processed into fragments and presented in combination with major histocompatibility complex (MHC) molecules on the surface of APCs. B cells can also recognize antigens; they have receptors that bind to specific ones. When B cells bind an antigen, it is internalized, processed, and then presented on their surface in association with MHC class II molecules.
Antigen processing involves the enzymatic breakdown of antigens into smaller pieces, which are then brought to the surface of the APC enclosed within the peptide-binding cleft of an MHC molecule. This specific complex of antigen fragment and MHC molecule is what T cell receptors actually recognize.
The immune system is thus highly specialized and efficient at identifying foreign bodies, and this specificity forms the basis of the body's ability to mount an immune response against pathogens, cancer cells, and transplanted organs.