When a foreign disease-causing agent (pathogen) enters the body, a protective system known as the immune system comes into play. This system consists of a complex network of organs and cells that can recognize the pathogen and mount an immune response against it.
Any substance capable of generating an immune response is called an antigen or an immunogen. Antigens are not the foreign bacteria or viruses themselves; they are substances such as toxins or enzymes that are produced by the microorganism. In a typical immune response, certain cells known as the antigen-presenting cells trap the antigen and present it to the immune cells (lymphocytes). The lymphocytes that have receptors specific for that antigen binds to it. The process of binding to the antigen activates the lymphocytes and they secrete a variety of cytokines that promotes the growth and maturation of other immune cells such as cytotoxic T lymphocytes. The cytokines also act on B cells stimulating them to divide and transform into antibody secreting cells. The foreign agent is then either killed by the cytotoxic T cells or neutralized by the antibodies.
The process of inducing an immune response is called immunization. It may be either natural, i.e., acquired after infection by a pathogen, or, the immunity may be artificially acquired with serum or vaccines.
In order to make vaccines for immunization, the organism, or the poisonous toxins of the microorganism that can cause diseases, are weakened or killed. These vaccines are injected into the body or are taken orally. The body reacts to the presence of the vaccine (foreign agent) by making antibodies. This is known as active immunity. The antibodies accumulate and stay in the system for a very long time, sometimes for a lifetime. When antibodies from an actively immunized individual are transferred to a second non-immune subject, it is referred to as passive immunity. Active immunity is longer lasting than passive immunity because the memory cells remain in the body for an extended time period.
Immunizations are the most powerful and cost-effective way to prevent infectious disease in children. Because they have received antibodies from their mother's blood, babies are immune to many diseases when they are born. However, this immunity wanes during the first year of life. Immunization programs, therefore, are begun during the first year of life.
Each year in the United States, thousands of adults die needlessly from vaccine-preventable diseases or their complications. Eight childhood diseases (measles, mumps, rubella, diphtheria, tetanus, pertussis, Hemophilus influenzae type b, and polio) are preventable by immunization. With the exception of tetanus, all the other diseases are contagious and could spread rapidly, resulting in epidemics in an unvaccinated population. Hence, vaccinations are among the safest and most cost-efficient public health measures. Vaccinations against flu (influenza), hepatitis A, and pneumococcal disease are also recommended for some adolescents and adults. The vaccines indicated for adults will vary depending on lifestyle factors, occupation, chronic medical conditions and travel plans.
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