Final answer:
B cells and T cells recognize foreign antigens through specific antigen receptors on their plasma membrane. B cells bind directly to antigens with B cell receptors, while T cells recognize processed antigens presented on MHC molecules. The interaction resembles a key fitting into a lock mechanism.
Step-by-step explanation:
B cells and T cells are capable of recognizing foreign antigens because they have specific antigen receptors in their plasma membrane. These receptors allow for the immune system's exquisite ability to detect and respond to pathogens. B cells can bind directly to intact antigens through their B cell receptors (BCRs), which are membrane-bound monomeric forms of IgD and IgM antibodies on their surface. T cells, on the other hand, recognize processed antigens presented on the major histocompatibility complex (MHC) molecules by antigen-presenting cells (APCs).
The recognition process is akin to a key fitting into a lock, with the antigen acting as the key and the receptor as the lock. Only when the correct antigen matches the receptor can the immune cell be sensitized and activated to fight the pathogen that contains the antigen.