It is a fair, even-handed, noble adjustment of things, that while there is infection in disease and sorrow, there is nothing in the world so irresistibly contagious as laughter and good-humour.
The immune system basically describes the collection of biological processes within an organism that serve to protect it against disease by identifying and killing pathogens.
We have a special type of immune response known as the adaptive immune response that allows the immune system to recognize and remember specific pathogens (to generate immunity), and to mount stronger attacks each time the pathogen is encountered. Phagocyte cells in the blood are able to engulf antigens (foreign cells/proteins) and display them on their cell surface (defects in this can lead to chronic granulomatous disease). Other cells found in the blood, known as T-lymphocytes (defects in these ca lead to severe combined immunodeficiency syndrome) then recognise these foreign proteins and release chemicals to further break down this foreign material and also release additional molecules that attract and stimulate the division of further immune cells. These antigens also bind to the cell surface of another type of immune cell, B-lymphocytes (B-cells), causing these cells to divide and produce antibodies (immunoglobulins; Ig). Antibodies then function to destroy antigens by causing them to precipitate or clump, and can in addition initiate further pathways that cause the destruction of the antigen and stop the antigen from entering host cells. Finally, some of the activated B cells and T cells then go on to become memory cells, dividing and reproducing the same antigen through the individual’s lifetime. These B and T lymphocytes form part of the immune response, as these memory cells are able to respond extremely quickly to an antigen if it is recognised from the previous infection.
Perhaps the earliest documented incidence of acquired immunity was during the plague of Athens in 430 BC. Here it was noted at the time that people who had recovered from a previous bout of the disease could nurse the sick without contracting the illness a second time. This forms the basis of vaccination whereby a substance is introduced into the body in order to produce immunity to a disease.
Although similar procedures may have been used in China and India in ancient times, the first recorded vaccination (Lat. vacca; cow), was by Edward Jenner in 1796 when he used cowpox to give immunity to smallpox after observing that dairymaids who had previously been sick with cowpox (a contagious disease that causes blisters on the cow's udder and on the milkmaid's hand) did not catch smallpox. Pasteur then further developed the use of artificial vaccines that could be used for a variety of diseases such as rabies by growing the virus in dogs and rabbits, then weakening it by drying the affected tissues, and finally reintroducing it into healthy animals. This serves to activate an immune response and form an immune memory so that when the animal comes in contact with the infectious form of the virus its immune system can quickly respond.
Before the advent of antibiotic drugs most babies born with inherited immune defects would have died early due to their susceptibility to infections and such cases would not have been easy to identify among the many normal infants who would have also died of infections. It was not until 1952 that the first immunodeficiency disease was described, by Dr. Colonel Ogden C. Bruton, when he reported the failure of a male child to produce antibodies due to a defective gene hindering B-cell growth. This is now known as Bruton's X-linked agammaglobulinemia, as the gene involved locates to the X chromosome. Dr Bruton also became the first physician to provide specific immunotherapy for this disorder by administering injections of antibodies to the patient who responded well to the treatment. Since then, many more diseases of antibody production have been described, where affected infants usually develop recurrent infections with pyogenic, or pus-forming, bacteria such as pneumonia-causing Streptococcus pneumoniae.
The most common inherited immunodeficiency is selective IgA antibody deficiency, which may be present in 1 in 400 people. While it seems to result in no specific symptoms it is found more often in people with chronic lung disease. Apart from IgA deficiency, the overall incidence of primary immunodeficiency is around 1 in 10,000.
While deficiencies in the immune system can lead to infections, an overactive immune system can lead to another class of diseases known as autoimmune diseases. This is characterised by a decreased ability of the immune system to distinguish foreign material from host cells resulting in the immune system attacking the individual’s own tissues.