-acquired and specific
-only works after an infection or vaccine
-specificity & memory

-lymphocyte development and differentiation
-the presentation of antigens
-the challenge of B & T cells by antigens
-B cells and the production and activities of antibodies
-T cell responses

-attachment to nonself or foreign markers
-binding to cell surface receptors that indicate self (i.e. MHC molecules)
-receiving and transmitting chemical messages to coordinate the response
-aiding in cellular development