Phagocytosis is where phagocytes provide a defence against pathogens that penetrate the surface of the host. Three types of phagocytes are Neutrophils - they are most abundant and short lived, they recognise and attack pathogens in tissue. Monocytes - these mature into macrophages which live longer then neutrophils and they consume large numbers of pathogens. Eosinophils - they are weakly phagocytic and they kill parasites.
Inflammation is the body response to infection or tissue injury. Inflammation is caused by damaged body cells releasing substances like histamine, most cells release this when they are damaged. The redness and heat of inflammation occurs from histamine induced dilation of blood vessels in the infected area. Histamine also causes the capillaries to become leaky which allows blood plasma and phagocytes into the tissue which causes swelling. The pain comes from increased pressure from the swelling and the action of leaked cells.
In damaged or infected tissue, complement proteins and other signals attract phagocytes, first neutrophiles and then monocytes which become macrophages. The macrophages engulf the invaders and debris so they are responsible for most of the healing associated with inflammation.
b) Antigens are organisms or molecules that are recognised by, or interact with, the immune system to initiate an immune response. The specific site on antigens that the immune system recognises are called antigenic determinants, this is a specific portion of a large molecule. A large antigen may have many different antigenic determinants on its surface each capable of being bound by a specific antibody or T cell. The host responds to the presence of an antigen by producing highly specific defence T cells or antibodies that correspond to the antigenic determents of the antigen. Each T cell and each antibody is specific for a single antigenic determinant.
There are two interactive immune responses, the humoral immune and cellular immune response. In the humoral immune response antibodies react with antigenic determinants of a foreign invaders blood, lymph and tissue fluid. Some antibodies are soluable and travel free in the blood and lymph, others exist as integral membrane proteins on specialised lymphocytes called B cells. The first time a specific antigen invades the body it may be detected by and bind to by a B cell whose membrane antibody recognises one of its antigenic determinants. This activated B cell forms a plasma cell that makes multiple copies of an antibody with the same specificity as the membrane antibody.
The cellular immune response is directed against an antigen that has become established within a cell of the host animal. It detects and destroys virus infected or mutated cells, without the use of antibodies. T cells within the lymph nodes, bloodstream and intercellular spaces carry this out. T cells have integral membrane proteins called T cell receptors, they have specific molecular configurations that bind to specific antigenic determinants, and once the T cell is bound to a determinant it initiates an immune response.
As mentioned earlier the activation of a B cell involves the binding of a particular antigenic determinant to the antibody protein on the surface of a B cell, for a B cell to develop into an antibody secreting plasma cells a T-helper cell (TH) must also bind to the same antigen on an antigen presenting cell.
T cells possesses specific surface receptors called glycoproteins. Like the immunoglobins T cell receptors include both variable and constant regions. The variable regions provide the specificity for reaction with a single antigenic determinant. When the T cells are activated by contact with a specific antigen determinant they proliferate and give rise to two types of effector cells.
Tc cells (cytotoxic T cells) - which recognise virus infected cells and kill them by causing them to lyse.
Th cells (Helper T cells) - which assist both the cellular and humoral immune systems.
c) When the body first encounters a particular antigen, a primary response is activated, and lymphocytes produce clones of effector and memory cells. The effctor cells destroy the invaders at hand and then die. After the first immune response to a particular antigen, subsequent encounters with the same antigen will result in a greater and more rapid production of antigen specific antibodies or T cells, this is called a secondary response. The benefits of this are that the immune system remembers the antigen so it can be immune to diseases. Opsonisation is the process of coating microorganisms with plasma proteins to increase there adherence to phagocytic cells in preparation for phagocytosis. The two main osponins are IgG antibody and the third component of complement (c3) which bind to the surfaces of microorganisms. The complement kills organisms by opsonisation followed by phagocytosis and intracellular killing. Agglutination is the clumping together of red blood cells or bacteria in response to a particular antibody and it occurs when antigens and the antibodies that are specific for them to come into contact with one another, so precipitation only occurs under correct conditions of antigens and antibodies. Autoimmunity relates to an immune response by the body aganst one of its own tissues, cells or molecules this can result indisease, for example multiple sclerosis causes damage to the nervous system. It involves both Tcell and B cell mediated attack on two major proteins in myelin the membrane that coats the nervous tissue. Causes of autoimmunity diseases are not known. Hypersensitivity is another type of condition that can arise when the immune system over reacts to a dose of antigen, although the antigen may present no danger to the host, the inappropriate immune response may produce inflammation and other symptoms that can cause illness or death. A consequence of organ or tissue transplantation caused by the transplant recipient's (host's) immune response to the transplanted organ/tissue which can damage or destroy it.The immune system distinguishes "self" from "foreign" by reacting to proteins on the surfaces of cells. It reacts against substances it recognizes as foreign (antigens). The presence of foreign blood or tissue in the body triggers an immune response that can result in blood transfusion reactions and transplant rejection when antibodies are formed against foreign antigens on the transplanted or transfused material.
