The lymphocytes or agranulocytes are of 2 types, T lymphocytes and B lymphocytes. These cells can be considered higher rank officials of the immune system and have a sophisticated mode of action. They are summoned by the chemical mediators produced by the neutrophils but reach the destination only after 4-7 days. Their importance is highlighted by the fact, that an absence of lymphocytes, which occurs in AIDS, leaves the body unprotected from infections and finally leads to early death. Thus, properly functioning lymphocytes are of prime importance in the sustenance of life.
The B lymphocytes are present in lymph nodes, and upon finding the relevant antigens, get activated and transform themselves to plasma cells. Plasma cells secrete large quantities of antibodies corresponding to the antigen found by the B lymphocytes. These antibodies circulate in blood and attach to all infecting organisms and cells present in the body having the concerned antigen. An antibody coated cell or organism is destroyed by the complement system, a system specialized in killing of cells marked with antibodies. The phenomenon of secreting antibodies and protecting the body from an infection which has spread from its primary site, is called humoral immunity. However, if the control over antibody secretion is lost, like in multiple myeloma, large quantities of non-specific antibodies are produced by plasma cells, which lead to widespread destruction of the organs of the body. Hence, proper regulation of humoral immunity is of paramount importance in preventing self-destruction (autoimmunity).
The T lymphocytes on the other hand, are subjected to rigorous controlled training in an organ called the thymus (hence the name, T lymphocytes). This helps them recognize all the tissues of one’s own body. Thus, they can easily differentiate any foreign antigen. Their response to an overwhelming infection, is in the form of multiplication of the particular cell family concerned with destruction of the particular foreign antigen. They not only destroy the antibody coated cells but also stimulate activation and formation of plasma cells by B lymphocytes. They secrete substances called interleukins, which promote plasma cells to produce large quantities of antibodies. In viral infections where, all other types of cells become ineffective, they take the decisions regarding killing of infected cells to destroy the viruses, and so on. The T lymphocytes are highly specialized and trained cells, which intelligently tackle infections by recognising and multiplication of a specific lineage of cells, which perform higher immune functions. Thus, the immunity conferred by these cells is called as cell mediated immunity.
Retaining the memory of an infecting agent forms an integral part of lymphocytes. Memory T lymphocytes and memory B lymphocytes retain the antigenic information of prior infections. During infections by the same organisms, these cells engage in the immune response much earlier and efficiently destroy the infection sometimes even before the granulocytes. The lymphocytes are slow acting but their actions are sustained even after the infectious threat is completely neutralized. Hence, the changes produced by their action is called as chronic inflammation.
The difference between granulocytes and lymphocytes is the specificity of their action. While granulocytes have action limited to the primary site only, lymphocytes on the other hand act throughout the body. Moreover, their prolonged action as well as memory ensures a complete removal of abnormal cells or infection and prevents recurrence of the disease.