IMMUNOCHEMISTRY

Immunochemistry is the study of the chemistry of immune responses.

An immune response is a reaction caused by the invasion of the body by an antigen. An antigen is a foreign substance that enters the body and stimulates various defensive responses. The cells mainly involved in this response are macrophages and T and B lymphocytes. A macrophage is a large, modified white blood cell. Before an antigen can stimulate an immune response, it must first interact with a macrophage. The macrophage engulfs the antigen and transports it to the surface of the lymphocytes. The macrophage (or neutrophil) is attracted to the antigen by chemicals that the antigen releases. The macrophage recognizes these chemicals as alien to the host body. The local cells around the infection will also release chemicals to attract the macrophages; this is a process known as chemotaxis. These chemicals are a response to the infection. This process of engulfing the foreign body is called phagocytosis, and it leads directly to painful swelling and inflammation of the infected area.

Lymphocytes are also cells that have been derived from white blood cells (leucocytes). Lymphocytes are found in lymph nodes, the spleen, the thymus, bone marrow, and circulating in the blood plasma. Those lymphocytes that mature inside mammalian bone marrow are called B cells. Once B cells have come into contact with an antigen, they proliferate and differentiate into antibody secreting cells. An antibody is any protein that is released in the body in direct response to infection by an antigen. Those lymphocytes that are formed inside the thymus are called T lymphocytes or T cells. After contact with an antigen, T cells secrete lymphokines—a group of proteins that do not interact with the antigens themselves, instead they stimulate the activity of other cells. Lymphokines are able to gather uncommitted T cells to the site of infection. They are also responsible for keeping T cells and macrophages at the site of infection. Lymphokines also amplify the number of activated T cells, stimulate the production of more lymphokines, and kill infected cells. There are several types of T cells. These other types include T helper cells that help B cells mature into antibody-secreting cells, T suppresser cells that halt the action of B and T cells, T cytotoxic cells that attack infected or abnormal cells, and T delayed hypersensitivity cells that react to any problems caused by the initial infection once it has disappeared. This latter group of cells are long lived and will rapidly attack any remaining antigens that have not been destroyed in the major first stages of infection.

Once the antibodies are released by the B and T cells, they interact with the antigen to attempt to neutralize it. Some antibodies act by causing the antigens to stick together; this is a process known as agglutination. Antibodies may also cause the antigens to fall apart, a process known as cell lysis. Lysis is caused by enzymes known as lytic enzymes that are secreted by the antibodies. Once an antigen has been lysed, the remains of the antigen are removed by phagocytosis. Some antigens are still able to elicit a response even if only a small part of the antigen remains intact. Sometimes the same antibody will cause agglutination and then lysis. Some antibodies are antitoxins, which directly neutralize any toxins secreted by the antigens. There are several different forms of antibody that carry out this process depending upon the type of toxin that is produced.

Once antibodies have been produced for a particular antigen they tend to remain in the body. This provides immunity. Sometimes immunity is long term and once exposed to a disease we will never catch the disease again. At other times, immunity may only be short lived. The process of active immunity is when the body produces its own antibodies to confer immunity. Active immunity occurs after an initial exposure to the antigen. Passive immunity is where antibodies are passed form mother to child through the placenta. This form of immunity is short lived. Artificial immunity can be conferred by the action of immunization. With immunization, a vaccine is injected into the body. The vaccine may be a small quantity of antigen, it may be a related antigen that causes a less serious form of the disease, it may be a fragment of the antigen, or it may be the whole antigen after it has been inactivated. If a fragment of antigen is used as a vaccine, it must be sufficient to elicit an appropriate response from the body. Quite often viral coat proteins are used for this. The first vaccine was developed by Edward Jenner (1749–1823) in 1796 to inoculate against smallpox. Jenner used the mild disease cowpox to confer immunity for the potentially fatal but biochemically similar smallpox.

Within the blood there are a group of blood serum proteins called complement. These proteins become activated by antigen antibody reactions. Immunoglobulin is an antibody secreted by lymphoid cells called plasma cells. Immunoglobulins are made of two long polypeptide chains and two short polypeptide chains. These chains are bound together in a Y-shaped arrangement, with the short chains forming the inner parts of the Y. Each arm of the Y has specific antigen binding properties. There are five different classes of immunoglobulin that are based on their antigen-binding properties. Different classes of immunoglobulins come into play at different stages of infection. Immunoglobulins have specific binding sites with antigens.

One class of compounds in animals has antigens that can be problematical. This is the group called the histocompatibility complex. This is the group of usually surface proteins that are responsible for rejections and incompatibilities in organ transplants. These antigens are genetically encoded and they are present on the surface of cells. If the cells or tissues are transferred from one organism to another or the body does not recognize the antigens, it will elicit a response to try to rid the body of the foreign tissue. A body is not interested where foreign proteins come from. It is interested in the fact that they are there when they should not be. Even if an organ is human in origin, it must be genetically similar to the host body or it will be rejected. Because an organ is much larger than a small infection of an antigen when it elicits an immune response, it can be a greater problem. With an organ transplant, there can be a massive cascade reaction of antibody production. This will include all of the immune responses of which the body is capable. Such a massive response can overload the system and it can cause death. Thus, tissue matching in organ transplants is vitally important. Often, a large range of immunosuppressor drugs are employed until the body integrates a particular organ. In some cases, this may necessitate a course of drugs for the rest of the individuals life. Histocompatibility problems also exist with blood. Fortunately, the proteins in blood are less specific and blood transfusions are a lot easier to perform than organ transplants. The blood-typing systems that are in use are indications of the proteins that are present. If blood is mixed from the wrong types, it can cause lethal clotting. The main blood types are A, B, O, and AB. Group O individuals are universal donors, they can give blood to anyone. Group AB are universal recipients because they can accept blood from anyone. Type A blood has A antigens on the blood cells and B antibodies in the plasma. The combination of B antibodies and B antigens will cause agglutination. There are also subsidiary blood proteins such as the rhesus factor (rh) that can be positive (present) or negative (absent). If only small amounts of blood are transfused, it is not a problem due to the dilution factor.

Immunochemistry is the chemistry of the immune system. Most of the chemicals involved in immune responses are proteins. Some chemicals inactivate invading proteins, others facilitate this response. The histocompatibility complex is a series of surface proteins on organs and tissues that elicit an immune response when placed in a genetically different individual.