By Dr Ananya Mandal, MD
The immune system of the body functions to protect it against invasion by germs and microbes. When an individual is vaccinated against a disease or an infection, his or her immune system is prepared to fight the infection.
Once vaccinated, when the person is exposed to the bacterium that causes it, the body gears up to fight off the infection. This is a mild episode that often goes unnoticed by the sufferer.
Vaccines take advantage of the body’s natural ability to learn how to eliminate almost any disease-causing germ, or microbe, that attacks it. Once vaccinated the body “remembers” how to protect itself from the microbes it has encountered before.
The immune system
The immune system is a complex interplay of cells and organs in the body that have evolved over centuries in all species to fight off infectious microbes. These cells are akin to an army with each type of cell specialized and designed to fight disease in a particular way.
The first encounter of the invading cells is with patrolling white blood cells called macrophages (literally meaning “big eaters”). These macrophages eat up and kill as many of the viruses and bacteria as they can.
The macrophages recognize these foreign invaders by specific proteins over the coat of the organism. This differentiates from the cells of the person’s own body as well so that the macrophages do not go about attacking the body’s own cells.
The macrophages alert the immune system to attack the invaders. Every microbe carries its own unique set of antigens. After the invasion, the microbes are ingested by the macrophages the antigens are saved. These antigens are markers of the particular germ that is carried back to other immune cells for them to “recognize” and “remember” for future use. This is done in the lymph nodes.
Within the lymph nodes are two major kinds of lymphocytes, T cells and B cells. These are the actual fighters.
T cells function either offensively or defensively. The offensive T cells do not attack the virus or microbe directly but act by releasing chemicals against the recognized antigen. These are called cytotoxic T cells or killer T cells. These can also sense the body’s cells that have been infected and are harbouring the microbes.
The killer T cells latch onto these cells and release chemicals that destroy the infected cells and the viruses inside.
Another type of T cells are the helper T cells. These are defenders. They secrete chemical signals that direct the activity of other immune system cells. Helper T cells assist in activating killer T cells, and helper T cells also stimulate and work closely with B cells.
The whole activity of the T cells is called cellular or cell-mediated immune response.
B cells are important in secreting and manufacturing antibodies against the specified antigens of the invading microbes. Antibodies usually work by first latching on to the recognized antigen and then sticking to and coating the microbe.
Antibodies and antigens fit together like pieces of a jigsaw puzzle. Each antibody is specific for each antigen and the body has records of thousands of these antibodies developed against antigens exposed. As a response to an infection millions of new B cells are made.
The antibodies from the B cells circulate throughout the body in blood until they encounter the virus or microbe and kill the organism. Once the antibodies bind to the antigens of the invaders, the macrophages and other killer cells find it simpler to look for the tagged cells and kill them.
The work of B cells is called the humoral immune response, or simply the antibody response. The B cell forms memory cells that remember the infection for life and prepares the body for arresting the infection next time around.
Vaccines and immunity
The goal of most vaccines is to stimulate the humoral response. The body is made to remember the antigens of the specific organism by inoculation of the weakened, killed or part of the organism. Many infectious microbes can be defeated by antibodies alone, without any help from killer T cells.
Vaccines and the community
Once the immune system is trained to resist a disease, the person becomes immune to it. Before the advent of vaccination, the person had to acquire the infection, suffer the symptoms and risk the complications that could be fatal in order to become immune to it. This type of immunity against an illness is called naturally acquired immunity. In addition, if the disease is contagious it may also be passed on to family members, friends, or others who come into contact.
Vaccines, which provide artificially acquired immunity, are a much safer way to become immune. Vaccines can prevent a disease from occurring in the first place and also decrease the risk of complications and risk of transmission. It is much cheaper to prevent a disease than to treat it. So vaccines protect not only individuals, but entire communities. That is why vaccines are vital to the public health goal of preventing diseases.
If a critical number of people within a community are vaccinated against a particular illness, the entire group becomes less likely to get the disease. This protection is called community immunity, or herd immunity. On the other hand, if a critical number of people in a community do not get vaccinations, diseases can reappear.
In 1974, the Japanese government stopped vaccinating against pertussis because of public concern about the vaccine’s safety and due to the decline in the deaths caused by whooping cough. Five years later, a pertussis epidemic in Japan affected 13,000 people and killed 41. Similarly due to low measles vaccine uptake a measles outbreak occurs in US in 1989. The outbreak resulted in more than 55,000 cases of measles and 136 measles-associated deaths.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)
Last Updated: Oct 14, 2012