How Vaccines Work
The immune system is a complex system of interacting cells whose primary purpose is to identify foreign substances, such as microorganisms and cancer cells, and to develop a defense, or immune response, against them. The immune response can be sub-divided into non-specific or innate immunity and specific or acquired immunity.
Innate immunity is present at all times in healthy individuals and consists of a series of mechanical and chemical barriers to prevent harmful substances from entering the body and specialized chemicals and cells to destroy foreign substances within the body. These non-specific mechanisms provide the first line of defense against disease-causing microorganisms until a specific immune response can develop.
Specific immunity only develops after the first exposure to a foreign substance or agent and it takes days to weeks to fully mature. On subsequent contact with the same substance or agent, the previously acquired immunity enables the body to specifically recognize or "remember" the substance or agent and react with a more rapid and stronger immune response to it. Immunologic memory is often long-lived and may give life-long protection against re-infection with infectious agents. The specific immune system can be stimulated to produce antibody and cellular immunity either by exposure to the natural disease or by administration of a vaccine.
Immunity mediated by soluble proteins, such as antibodies and complement, found in the blood and secretions is referred to as "humoral immunity". Antibodies belong to a family of protein molecules known as immunoglobulins that can specifically recognize foreign substances or antigens. Attachment of antibody to antigens on an infecting microorganism marks the cell or virus particle as foreign and enables the immune system to destroy or neutralize the invader. Foreign particles coated with antibody are said to be opsonized or "prepared for eating" and can be more readily identified, ingested and killed by specialized cells called phagocytes. This effect can be strengthened if other components of the humoral immune system, called complement, bind to the antibody attached to an antigen. The binding of complement to the surface of microbial cells can also directly cause cell death and cell disruption, in the absence of phagocytic cells, by punching holes in the cell membrane. Although this process can occur in a limited form without antibodies, as part of the innate immune system, the presence of bound antibodies further enhances the process. The attachment of antibodies to microorganisms can also block infection by preventing the interaction of microorganisms, or their products, with host cells. This process can hinder the colonization of mucosal surfaces by microbes, prevent infectious agents from invading body cells and neutralize bacterial toxins before they can poison the host.
Cellular immunity is effected by white blood cells known as lymphocytes of which there are two major classes, B and T lymphocytes. B cells are key players in the development of humoral immunity. The B cells have specific immunoglobulin antigen binding receptors on their cell surface through which they can bind antigen. This event, together with co-stimulatory signals provided by antigen presenting cells and T cells, activates the B cells, which increase in number before developing into plasma cells or memory cells. Plasma cells secrete antibodies specific for the antigen initially recognized by the B cell.
Memory cells enable the body to react with a more rapid antibody response on re-encountering the antigen. T cells are involved in many ways in the development and regulation of immune responses. One subset of T cells, called T helper cells, assists in the development of the humoral immune response through interactions with B cells. Another subset of T cells, cytotoxic T cells, are able to specifically identify and kill cells infected with intracellular bacteria or viruses as well as cancer cells. T cells are also involved in inducing inflammation and in inhibiting certain immune responses.
Vaccines interact with the immune system to produce a specific immune response, which is often identical to that produced by the natural infection but without the hazards associated with the disease. The widespread use of vaccines has led to marked reductions in the incidence of numerous infectious diseases, the near elimination of polio and the eradication of smallpox. Vaccines prevent illness or death for millions of individuals every year and vaccination is undoubtedly one of the greatest success stories in the history of medicine.