Immunity is derived from Latin word – immunis, meaning free from burden. Burden refers to diseases caused by micro-organisms or their toxic products.
Therefore, immunity is defined as the resistance exhibited by the host towards infection caused by micro-organisms or their product (toxins).
This is based on the property of self and non-self recognition, which means immunity is carried out by the process of recognition and disposal of non-self or materials that enter the body.
Basically, immunity is of two types: Innate (non-specific) and Acquired (specific)
An individual is born with innate immunity. Genetic and constitutional make up of an individual contributes to innate immunity. This kind of immunity has no relationship with previous bacterial infection and immunization. It acts as a first line of defense against infections, micro-organisms, their products before they cause disease.
Different mechanisms through which innate or non-specific immunity is expressed includes
- Anatomical and physical barriers (like skin, mucus membrane)
- Physiological and chemical barriers (like human milk, gastric juice, saliva)
- Biological barriers (for example phagocytosis)
- General barriers (that could be age-related, racial, or individual immunity)
Acquired immunity, as the name suggests, is developed later in life. It is also referred to as Adaptive immunity as it is highly adaptive and has four characteristic features:
- antigen specificity,
- immunological memory, and
- recognition of self from non-self
Since the components of acquired immunity such as antibodies and T-cells are specific to particular micro-organisms, it is also called as Specific immunity.
Acquired or adaptive or specific immunity is of two types; Active (where individual’s own immune system produces antibodies), and Passive (wherein the antibodies produced in another person or an animal are transferred to produce immunity)
Both, active and passive types could either be natural or artificial;
- Natural active immunity (wherein immunity is provided by natural exposure or infection),
- Artificial active immunity (wherein antigens are deliberately introduced through a vaccine)
- Natural passive immunity (wherein antibodies from mother are transferred to her child across the placenta and through her milk),
- Artificial passive immunity (where antibodies produced in or by another person or an animal are injected)
Having known about the immunity and types of immunity, let us try to understand another term, which is trending a lot, presently, and that is Herd Immunity.
When a large part of a population of an area is immune to an infectious disease, this provides indirect protection to those who are not immune to the disease, since the infective agents have nowhere to spread more. Hence it is called as population immunity, or community immunity, or herd immunity, or herd protection).
Herd immunity protects at-risk populations including babies and those having weak immune system and can’t get resistance on their own.