The immune system protects against pathogenic (disease-causing) organisms (e.g. bacteria, viruses, fungi, parasites), particles such as toxins and pollutants & cells (e.g. cancer cells). Trigger particles for the immune response are called antigens and these are recognised by antigen receptor sites on the surface of immune cells.
There are 2 elements to the immune system – innate (non-specific defence) and adaptive (specific defence) immunities. Innate immunity is the first line of immediate defence against entry into the body, having a generalised action but no “memory” of the attack. It uses physical barriers such as skin, antimicrobial chemicals, such as acid, interferons & complement and involves white blood cells (leucocytes) namely phagocytes, cytotoxic, and natural killer (NK) cells and cells involved in the inflammatory response.
Adaptive immunity recognises and responds to the epitopes on the antigen surface by using antigen receptors. There are more than 100 million different naturally occurring antigen/pathogens. The antigenic epitopes correspond to a huge diversity of leucocytes called small lymphocytes. For each antigen there is a corresponding small lymphocyte with an antigen receptor specific to that antigen. Small lymphocytes comprise T- and B- cells. Both are made in the bone marrow, but B-cells continue to mature there whereas T-cells then mature in the thymus. Both respond to one type of antigen & no other.
At birth we possess small numbers of all the types of small lymphocyte needed to counteract any known antigen, but the numbers are too small to overcome a pathogenic attack immediately On meeting an antigen/pathogen for the first time the number of specific lymphocytes increase in number (clonal expansion) and develop into two types of cell, defence cells which destroy the pathogen and memory cells that remain in the body after the pathogenic attack has been overcome (immunological memory). This primary response is slow to develop meaning that the patient develops symptoms of the illness. On meeting the same antigen/pathogen for a second time, the memory cells are quick to respond and clonal expansion occurs rapidly giving a much greater immune response and so the pathogen is quickly overcome and the patient does not develop symptoms of the disease. The patient is immune.
B-cells are fixed in the lymphatic tissue & on clonal expansion generate plasma cells that produce antibodies which are proteins called immunoglobulins (IG). Each antibody recognises, binds with and destroys a specific antigen. T-cells are of two types, cytotoxic T cells which destroy cancer cells and cells that are infected by a virus and B helper cells that act in many ways to support cells in both the innate and adaptive immune systems.
The major histo-compatibility complex (MHC) is a collection of molecules on the surface of body cells that is unique to each person and are responsible for the immune cells recognising cells as being “self” & “non-self”. Transplants between different people, are rejected because the immune system recognises them as being “non-self” and immunosuppressive drugs are needed to prevent the transplant being attacked and destroyed. In pregnancy foetal cells are not generally rejected. However, in the case of a rhesus negative mother bearing a rhesus positive baby, rhesus positive antigens may cross the placenta from baby to mother whose immune system will produce antibodies which will enter the baby and cause the destruction of foetal red blood cells. Autoimmune disease is where the immune system falsely begins to recognise its own cells as foreign and starts to destroy them (e.g. Type 1 diabetes, rheumatoid arthritis, coeliac disease and multiple sclerosis).