There are two large groups of mechanisms that protect us against pathogenic organisms and processes. These can be divided into innate defense mechanisms and adaptive defense mechanisms. Our innate defense mechanisms are present at birth, ready to act. Our adaptive mechanisms have to be trained by exposure to potentially injurious organisms and substances. The adaptive immune mechanisms are further subdivided into mechanisms of humoral immunity and the those of cellular immunity ( cell-mediated immunity). In humoral immunity processes, protection is provided by lymphocytes that circulate in blood plasma and lymph. Lymphocytes are able to bind to extracellular targets such as bacteria, free viruses, bacterial toxins, They then secrete antibodies to inactivate these targets and identify them for destruction by phagocytes or complement. In cell-mediated immunity, the cells ( lymphocytes ) themselves--not their antibodies provide the defense against pathogens. The important actors in cell-mediated immune responses are T-cytotoxic, T-Helper, and NK cells. These cells are also found in lymph nodes and spleen .
Work Step by Step
The functional cells of humoral immunity are lymphocytes-- t-lymphocytes, b-lymphocytes . Macrophages are also have a role in humoral immunity. These lymphocytes originate in bone marrow and mature in bone marrow. They protect against microbes, and other pathogens( and their toxins). B-lymphocytes are found in spleen and lymph nodes . When B -lymphocytes come in contact with antigens for the first time, their immune response is relatively slow. But these cells develop immune memories for specific antigens and when they meet them again the secondary immune response is very rapid. B-cells produce several types of surface receptors-- CD40 , CD21-- and produce several classes of antibodies like IgM, and IgD antibodies. After the first encounter with a specific antigen, a B- cell forms a clone of antibody secreting cells. These plasma cells can react very rapidly to their specific targets without the mediation of major histocompatibility complex (MHC/HLA) antigens. Specific immunity has another equally valuable arm, namely, cell-mediated immunity. This type of immune reactions depend on T-lymphocytes. These lymphocytes develop in bone marrow, but unlike B-lymphocytes they mature in the thymus. They protect against viruses, parasites, bacteria and cancer cells No antibodies are formed in cell-mediated immunity ; targets are acted on by cytokines secreted by T-cells. The targets of cell-mediated immunity may be tissue cells infected by viruses, parasites, cancer cells, or cells of foreign graft tissues. Pathogens are usually destroyed intracellulary in a process that involves Type II MHC molecules.