NCERT Solutions for Biology Class 12 Types of Vaccines
Biology Class 12: In the previous article, we have discussed What are Vaccines? Now we will discuss types of vaccines.
Vaccines are dead or inactivated organisms or purified products derived from them.
There are several types of vaccines in use. These represent different strategies used to reduce the risk of illness while retaining the ability to induce a beneficial immune response.
Some vaccines contain inactivated, but previously virulent, micro-organisms that have been destroyed with chemicals, heat, or radiation. Examples include the polio vaccine, hepatitis A vaccine, rabies vaccine, and some influenza vaccines.
Some vaccines contain live, attenuated/ weakened microorganisms. Many of these are active viruses that have been cultivated under conditions that disable their virulent properties, or that use closely related but less dangerous organisms to produce a broad immune response. Although most attenuated vaccines are viral, some are bacterial in nature. Examples include the viral diseases yellow fever, measles, mumps, and rubella, and the bacterial disease typhoid. The live Mycobacterium tuberculosis vaccine developed by Calmette and Guérin is not made of a contagious strain but contains a virulently modified strain called “BCG” used to elicit an immune response to the vaccine. The live attenuated vaccine containing strain Yersinia pestis EV is used for plague immunization. Attenuated vaccines have some advantages and disadvantages. They typically provoke more durable immunological responses and are the preferred type for healthy adults. But they may not be safe for use in immunocompromised individuals, and may rarely mutate into a virulent form and cause disease.
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Toxoid vaccines are made from inactivated toxic compounds that cause illness rather than the micro-organism. Examples of toxoid-based vaccines include tetanus and diphtheria. Toxoid vaccines are known for their efficacy. Not all toxoids are for micro-organisms; for example, Crotalusatrox toxoid is used to vaccinate dogs against rattlesnake bites.
Rather than introducing an inactivated or attenuated micro-organism to an immune system (which would constitute a “whole-agent” vaccine), a fragment of it can create an immune response. Examples include the subunit vaccine against Hepatitis B virus that is composed of only the surface proteins of the virus (previously extracted from the blood serum of chronically infected patients, but now produced by recombination of the viral genes into yeast), the virus-like particle (VLP) vaccine against human papillomavirus (HPV) that is composed of the viral major capsid protein, and the hemagglutinin and neuraminidase subunits of the influenza virus. Subunit vaccine is being used for plague immunization.
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Certain bacteria have polysaccharide outer coats that are poorly immunogenic. By linking these outer coats to proteins (e.g., toxins), the immune system can recognize the polysaccharide as if it were a protein antigen. This approach is used in the Haemophilus influenzae type B vaccine.
Vaccines and the Immune System
Thus, we see that the immune system recognizes vaccine agents as foreign, destroys them, and “remembers” them. When the virulent version of an agent is encountered, the body recognizes the protein coat on the virus, and thus is prepared to respond, by:
(1) Neutralizing the target agent before it can enter cells, and
(2) Recognizing and destroying infected cells before that agent can multiply to vast numbers.
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