The Defense System
The immune system is a complex system made up of many biological structures and processes within an organism that protect it from harmful pathogens.The body's immunity starts off with the first line of defense which includes protection via skin, mucous membranes, stomach acid and tears. Once a pathogen breaches any of these barriers the innate immune response begins its role during the first 12 hours of infection. After that the adaptive immune response begins its role.The innate immune system responses attack microbes indiscriminately and does not provide long-lasting immunity(6). It is comprised of different cells such as NK cells, macrophages and dendritic cells that attack all foreign things in the body and are not specific. The adaptive immune system has a stronger response and is more specific as it develops immunological memory of the pathogen. The cells involved in this subsystem are lymphocytes, either originating from the bone marrow (B-Cells) or thymus (T-Cells)(6). The B-cells become activated by specific interactions with pathogens and T-Cells. Once activated, these B-Cells begin producing antibodies which are are used to attack and kill the pathogen. In order for the B-cells to be activated they must bind to a specific T-Cell. Without T-Cells the adaptive immune response cannot be initiated(7).
Cells of Innate Immune System
NK Cells: These cells do not directly attack invading microbes, instead they destroy compromised host cells. Compromised host cells include tumor cells as well pathogen infected cells(6). The NK cells recognize these infected cell by detection of the lack of a MHC I (major histocompatibilty complex, displayed by most cells in the human body)(6).
Phagocytes: This word literally means "eating cell" as it engulfs pathogens. Phagocytic cells extend portions of their plasma membrane, wrapping the membrane around the pathogen. Lysosomes in cells contain acids that then lyse or kill the pathogen(7). There are three types of phagocytes; macrophages, neutrophils and dendritic cells. Macrophages are the largest of the phagocytes and move across capillary walls trying to find invading pathogens(6). Neutrophils are usually first to the site of the infection and contain neutrophil granules that contain a variety of toxic substances that kill/inhibit pathogens(6). The last type, dendritic cells, are present in tissues that are in contact with the external environment and are an example of antigen presenting cells just like macrophages.
Basophils/Eosinophils: Basophils are cells that release histamine which is important in defense against pathogens, as it increases blood flow to infection site allowing more white blood cells to come in and attack(6,7). Eosinophils are responsible for controlling the amount of histamine released(7).
Antigen Presenting Cells: These specialized cells internalize the pathogen via phagocytosis and then express parts of its antigen on the MHC II which is located on the cell's surface. These antigens are the recognized by T-Cells and trigger the production of antibodies and release of cytokines(6). These include the macrophages, dendritic cells and B-Cells(6).
Cells of the Adaptive Immune System
T-Cell: There are two types of T-cells; Helper T-cells (CD4) and cytotoxic T-cells (CD8). Helper T-cells have T-cell receptors which bind to the MHC II with the antigen on phagocytic cells that have engulfed pathogens. However this can only occur if the variable portion of the T-cell receptor is specific to the MHC II and antigen complex(7). After the binding occurs the T-cell becomes activated and differentiates into either memory T-cells or effector T-cells. Memory T-cells stay in the body for many years in case the same pathogen attacks again. Effector T-cells raise the alarm and release cytokines which enter other lymphatic cells signalling them to divide more often(7). They also help activate B-cells which secrete antibodies .Cytotoxic T-cells recognize viral epitopes presented on MHC I complex and deliver toxins to the infected host cell causing cell death.
B-cell: This lymphatic cell has hundreds of the same protein complexes surrounding it (but each B-cell has a different type of variable portion). B-cells directly bind to pathogens only, if the variable portion of their membrane bound proteins is specific to an epitope on the pathogen(7). The B-cell then engulfs that pathogen, lyses it and places parts of its antigen on its MHC II complex, which is recognized by the activated helper T-cells(7). The T-cell then binds to the B-cell, alongside releasing cytokines which in turn activates the B-cell. Then the B-cell differentiates into memory B-cells and plasma cells, memory B-cells stay in the body for years in case of another infection(6). Plasma B-cells become antibody producing factories, and release these antibodies (which have the same variable portions as the original B-cell) into the bloodstream. These antibodies then bind to any pathogen they come in contact with which makes it easier for the phagocytes the engulf that pathogen(7).