Vaccination – Biology

Vaccination A vaccination is the injection of a killed microbe into the body in order to stimulate the immune system against the microbe, thereby preventing the disease. Vaccines are manufactured from the dead or attenuated bacteria, inactivated viruses, purified polysaccharides from bacterial walls, toxoids, and even recombinant DNA produced through genetic engineering. This is to immunize against viral diseases. A healthy immune system is able to recognize the invading bacteria or virus and so produces antibodies to destroy or disable them.

Only a small portion of the dead bacteria is generally required to stimulate the formation of antibodies against the whole bacteria. When an infection occurs the lymphocyte population responds. Both T-lymphocytes and B lymphocytes are formed by divisions of the stem cells in the bone marrow. The T-lymphocytes leave the bone marrow during development and differentiate in the thymus gland before circulating and storage in the lymph glands. T-lymphocytes carry out cell-mediated immunity, an immune response that doesn’t directly involve antibodies but does have a part in the activation of the B-lymphocytes.

Certain T-lymphocytes are effective against pathogens within host cells. While the T-lymphocytes leave the bone marrow before completing maturation, the B-lymphocytes complete maturation before leave the bone marrow to circulate around the body and be stored in the lymph nodes. If an antigen is found, humoral immunity takes place. This is when B-lymphocytes proliferate into plasma cells that secrete antibodies into the blood stream. Both T-lymphocytes and B-lymphocytes have molecules on the outer surface of their plasma membrane, enabling them to recognize antigens.

However, each lymphocyte has only one type of surface receptor and therefore can only recognize one type of antigen. As mentioned before, the lymphocyte population responds when an infection occurs by an increase in number and collection at the site of the invasion. When a specific antigen enters the body, B-lymphocytes recognize the particular antigen and bind to it. On binding with the B-lymphocyte, the antigen is taken into the cytoplasm by phagocytosis before being expressed on the B-lymphocyte’s plasma membrane.

Phagocytic cells of the body, including macrophages, engulf the antigens they encounter. This occurs in the plasma and lymph. Once these antigens are taken up, the macrophage presents them externally by attaching the antigen to their MHC antigens. MHC are glycoproteins that identify cells and each individual’s MHC is genetically determined therefore a feature we inherit. Lymphocytes of our own immune system have antigen receptors that recognize our own MHC antigens and differentiate these from foreign antigens detected in the body.

The process by which the antigen is presented by a macrophage through the use of MHC is known as antigen presentation. T-lymphocytes then come in contact with the macrophages and briefly bind to them, immediately activating the T-lymphocyte. Activated helper T-lymphocytes then bind to B-lymphocytes with the same antigen expressed on their plasma membrane and the activated T-lymphocyte sends a message to the B-lymphocyte to activate it.

Activated B-lymphocytes immediately divide very rapidly by mitosis forming a clone of cells known as plasma cells, which are packed with rough endoplasmic reticulum. The plasma cells mass produce and export antibodies from the B-lymphocytes through exocytosis. An antibody is a special protein called immunoglobulin and is made of four polypeptide chains held together by disulphide bridges, forming a molecule in the shape on a Y. The arrangement of the amino acid residues in the polypeptides that form the fork region makes it totally unique to the other antibodies.

It is this region that forms the highly specific binding site for the antigen. After these antibodies have tackled the antigens, they disappear from the blood and tissue fluid, along with the bulk of the specific B-lymphocytes and T-lymphocytes responsible for their formation. However, certain of these specifically activated B-lymphocytes and T-lymphocytes are retained in the body as memory cells. This is the basis of natural immunity. Benefits of Vaccinations * Dangerous diseases may be eradicated as was small pox in 1977. Vaccines prevent diseases that otherwise may result in unpleasant and sometimes life threatening conditions. Measles is a major cause of infant death in many less-developed countries as the vaccination is not available to them. * Long term disability from disease can be prevented such as disabilities in unborn children since the creation of rubella infections which are injected into pregnant women and can prevent deafness, blindness, brain damage, and heart disease. Dangers of Vaccinations In the worst cases which are extremely rare, vaccines can actually cause the disease they are designed to prevent. * Vaccines can cause occasional adverse reactions * Whooping cough vaccine may cause brain damage – As the incidence of a disease begins to fall dramatically due to an immunization programme, rare side-effects of the vaccine appearing in a minority can become unacceptable to the public. This leads to a serious loss of confidence in what are actually favourable treatments.