The resistance displayed by the host towards an injury caused by microbes and their products is known as immunity. It is the state comprising of adequate biological guards to keep away illness or disease and also the ability of the body to defy dangerous germs from piercing into it.

Immunity against infection is of different types;

Innate immunity is the resistance to infections which an individual possess by virtue of his genetic and constitutional make up. It is being obtained by all human beings because they belong to the human species. It may be considered at species, racial or individual level. It includes;

1. Specific
2. Non-specific
Specific and non-specific gears are involved in innate immunity. The specific which acclimatize the body to newly entering pathogens and non-specific involves the general resistance against an extensive array of pathogens.

Factors that influence the level of innate immunity are;

1. Age: the fetus inside the uterus is free from infections to some extent, but some pathogens like Rubella, Toxoplasma, etc. The susceptibility of prepubertal girls to gonorrheal infections is also common. Old persons are highly susceptible due to waning of their immune responses and other sickness like prostrate leading to urinary stasis.

2. Hormonal influences: endocrine disorders like diabetes mellitus, hypothyroidism, etc. are associated with an enhanced susceptibility to infections. Corticosteroids suppress the host’s resistance through hypersensitivity and holding back the antibody formation. The increased steroid level in pregnant ladies makes easily prone to various infections.

3. Nutrition: humoral and cell mediated immunity are repressed by malnutrition. Malarial parasites remain dormant in famine stricken places, but shows off fever when the body regains nourishment.

Acquired Immunity

The immunity that is acquired during the life time as a result of antigenic stimuli is the acquired immunity. It represents adaptive responses in the host by production of antibodies.

1. Active; it may be natural or artificial. Natural immunity develops after recovery from a particular disease. Some diseases like chicken pox provide lifelong immunity once cured. There is a latent period which is needed to develop the resistance.

2. Passive; the resistance that is passed onto an individual in a readymade form is termed as passive immunity. Preformed antibodies are administered without any antigenic stimulus. No latent period is required as it provides immediate effect soon after immunization.

Traditional vaccines come under passive immunity and are;

Bacterial vaccines:

1. Live attenuated vaccine; artificially grown microbes without infectious ability. Example; BCG vaccine for tuberculosis.
2. Inactivated vaccine; consists of heat killed or chemically treated microbes which become non-infectious. Example; cholera vaccine.
3. Subunit vaccines; preparations containing small splinters of organisms that are responsible for causing disease. Example; typhoid Vi antigen.
4. Toxoid; poisonous compounds from the microbes which are inactivated. Example; tetanus vaccine.

Viral vaccines:

1. Live, example; oral polio vaccine.
2. Killed, example; injectable polio vaccine
3. Subunit, example; Hepatitis B vaccine.

Immunity should be measured in order to test the resistance of the individual to a challenge by the pathogen. But it not practical as the challenge itself will the state of immunity, so accurate measurement is not possible. Statistical data can be collected by using large numbers of individuals.

Specific antibody demonstration is a simple method. This is achieved through neutralization ELISA, agglutination, precipitation, complement fixation, etc. In the case of cell mediated immunity; skin tests are done to detect delayed hypersensitivity.