WHAT IS IMMUNITY- Immunity refers to the body’s ability to prevent the invasion of pathogens. Pathogens are foreign disease-causing substances, such as bacteria and viruses, and people are exposed to them every day. An immune response is the body’s defense system to fight against antigens and protect the body.

When the body senses foreign substances (called antigens), the immune system works to recognize the antigens and get rid of them. Each second, human beings are exposed to bacteria, viruses and fungi, which try to enter the body to make it their permanent home. Therefore, to protect itself from these threats, the body has an effective, fast and intelligent system called: the immune system.

This system is composed of a network of cells, tissues and organs that coordinate the defense of the body against any threat. Without the help of this network, any small injury (such as a paper cut) could be lethal.

White blood cells, also called leukocytes, are the immune system heroes, as they are the ones that fight germs directly. They only represent 1% of the total blood cells, but play an important role. When a part of the body is under attack, the white blood cells are the ones that destroy the dangerous substance, also called antigen, and prevent the body from getting sick. This protection is called immunity. They are made in the bone marrow and are stored in the blood and lymphatic system. In fact, white blood cells do not live very long. Their lifespan ranges from 1 to 3 days, so the body is always making new ones.

TYPES OF IMMUNITY- Three types of immunity exist — innate, active and passive:

1. Innate immunity is general protection that a person is born with, including physical barriers (skin, body hair), defense mechanisms (saliva, gastric acid), and general immune responses (inflammation). This type of immunity is considered non-specific. Although the immune system does not know exactly what kind of antigen is invading the body, it can respond quickly to defend against any pathogen.

2. Active immunity [Acquired (adaptive)] occurs when our own immune system is responsible for protecting us from a pathogen. It is a type of immunity that develops from immunological memory. The body is exposed to a specific antigen and develops antibodies to that specific antigen. When next time said antigen invades, the body has a memory of the specific antigen and already has antibodies to fight it off.

Acquired immunity can occur from exposure to an infection, wherein a person gets a disease and develops immunity as a result. Acquired immunity also occurs from vaccination wherein the vaccine mimics a particular disease, causing an immune response in the vaccinated individual without getting them ill.

3. Passive immunity occurs when we are protected from a pathogen by immunity gained from someone else. It is the body’s capacity to resist pathogens by “borrowing” antibodies. For example, antibodies can be transferred to a baby from a mother’s breast milk, or through blood products containing antibodies such as immunoglobulin that can be transfused from one person to another like plasma therapy. The most common form of passive immunity is that which an infant receives from its mother. Passive immunity is temporary until the antibodies go, since the body has not produced the antibodies.

HERD IMMUNITY- ‘Herd immunity’, also known as ‘population immunity’, is the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection, thereby reducing the likelihood of infection for individuals who lack immunity. This means that even people who aren’t vaccinated, or in whom the vaccine doesn’t trigger immunity, are protected because people around them who are immune can act as buffers between them and an infected person.

Once herd immunity has been established for a while, and the ability of the disease to spread is hindered, the disease can eventually be eliminated. For example, this is how the world eradicated smallpox. However, natural herd immunity – achieved through infection rather than vaccination – can be challenging to induce through unchecked infection as there would be a very high rate of serious illness and death, with health systems overwhelmed well beyond their surge capacity, even in high-income countries. This is why herd immunity is generally pursued through vaccination programs.

The new coronavirus has a lower infection rate than measles, with each infected person passing it on to two or three new people, on average. This means that herd immunity should be achieved when around 60% of the population becomes immune to COVID-19. But herd immunity is vulnerable to the individuals who lack immunity, particularly those who choose not to get vaccinate, enjoys, free ride off the herd immunity created by those who are immune. As the number of free riders in a population increases, outbreaks of preventable diseases become more common and more severe due to loss of herd immunity.