If they achieve immune-competence within the bone marrow, they are known as B cells, or if in the thymus (also a primary lymphoid organ), they are known as T cells. Organized lymphoid tissue elsewhere is known as secondary lymphoid tissue, and includes lymph nodes, adenoids, tonsils and mucosa associated tissue (MALT). MALT includes bronchus associated lymphoid tissue (BALT), gut associated lymphoid tissue (GALT), naso-phayngeal associated lymphoid tissue (NALT), and uro-genital associated lymphoid tissue. These lymphoid organs receive antigens from the tissues and mucosal surfaces. Antigens that succeed in invading the blood stream are intercepted in the spleen.

Lymphocytes respond to presented antigens by the production of antibodies (by B cells), to be described later, or lymphokines (by T and B cells). These have many actions, including control of the adaptive immune response by secondary action on the participating cells, and, in the case of cytolytic T cells, in killing virally-infected host cells.

Lymphocytes possess receptors for these polypeptide antigens. The ability of a molecule or molecular configuration to induce an immune response is spoken of as immunogenicity, and the molecule as an immunogen. A molecule able to react with the ensuing antibody or T cell receptor is spoken of as an antigen. Some antigens, whilst able to react, are unable to induce, i.e. they lack immunogenicity and are known as haptens.