Types of Immunity
Immunity is the body’s ability to protect itself from infectious diseases. It has two main parts: Innate or Non-Adaptive immunity and Specific (Adaptive) immunity.
Innate Immunity
Innate immunity is the body’s first line of defence and is present from birth. It is not specific to any one pathogen. Instead, it protects against many common germs using general defence mechanisms. These include physical barriers like the skin, cells such as macrophages and neutrophils that engulf and destroy microbes, and antimicrobial substances produced by the body. Together, these components help prevent infection at an early stage.
In general, most of the microorganisms encountered by a healthy individual are readily cleared within a few days by defence mechanisms of the innate immune system.
Adaptive Immunity
Adaptive immunity can specifically recognize and destroy particular foreign microbes or substances (antigens). Unlike innate immunity, which is the same in everyone, adaptive immunity is unique to each individual and develops in response to specific antigens encountered.
The adaptive immunity having the four major ways to perform its actions;
Major Characteristics of Adaptive Immunity
- Antigenic Specificity: The immune system can recognize and distinguish very small differences between antigens. The components which participates in adaptive immunity are so specific, that they can differentiate between two protein molecules that differ by just a single amino acid and respond to them
- Diversity: The immune system is capable of producing a vast variety of recognition molecules. This diversity allows the adaptive immunity to identify and respond to billions of different foreign antigenic structures.
- Immunologic Memory: Once the immune system encounters an antigen and responds to it, it remembers that antigen. On second exposure of the same antigen, the immune response is faster and stronger and often providing long-term immunity against that particular antigen.
- Self / Non-self Recognition: The immune system normally responds only to foreign (nonself) antigens. It can distinguish between the body’s own molecules (self) and foreign substances, ensuring that immune responses are directed only against nonself antigens.
Comparison of Innate and Adaptive Immunity
| Factor | Innate Immunity | Adaptive Immunity |
|---|---|---|
| Response Time | Hours | Days |
| Specificity | Limited and Fixed | Highly Diverse and improves later with the immune response |
| Secondary Response | Same as Primary | Much rapid, effective and prolong as compared to primary response |
| Key Components | Skin, Mucous membranes, Lysozymes | B- Lymphocytes, T-Lymphocytes, B-Cell membrane bound antibodies, Class I and II MHC Molecules, various other immune cells and immune mediators |
Acquired or adaptive immunity is the type of immunity that develops in any individual’s body after exposure to any specific antigens. It is characterized by high specificity and immunological memory.
This form of immunity is broadly divided into active immunity and passive immunity.
Active Immunity
Active immunity is the type of immunity in which an individual’s own immune system produces antibodies in response to an antigen. Since the body actively participates in antibody formation, this immunity develops slowly but is usually long-lasting and may even be lifelong due to the formation of memory cells.
Natural Active Immunity
Natural active immunity develops when a person is naturally exposed to a disease-causing organism during an infection. The immune system recognizes the antigen, produces antibodies, and forms memory cells. As a result, that person becomes protected against future infections by the same pathogen.
Example: Chickenpox, measles.
Artificial Active Immunity
Artificial active immunity is produced when antigens are deliberately or intentionally introduced into the body through vaccination. Vaccines contain weakened, killed, or purified forms of pathogens that stimulate the immune system without causing disease. This helps the body to develop immunity safely.
Example: BCG vaccine (tuberculosis), polio vaccine, hepatitis B vaccine, and MMR vaccine etc.
Passive Immunity
Passive immunity is the type of immunity in which ready-made antibodies are transferred to an individual from another person or animal. Since the body does not produce these antibodies itself, passive immunity provides immediate protection but lasts for a short period, as no memory cells are formed.
Natural Passive Immunity
Natural passive immunity occurs when antibodies are transferred from a mother to her child. This protects the baby during early life when its own immune system is not fully developed.
Example: Transfer of IgG antibodies through the placenta before birth and IgA antibodies through breast milk after birth, which protects newly born babies from infections.
Artificial Passive Immunity
Artificial passive immunity is achieved by injecting preformed antibodies into a person to provide immediate protection, especially in emergencies. This type of immunity is commonly used when there is no time to wait for active immunity to develop.
Example:Anti-tetanus serum, rabies immunoglobulin, diphtheria antitoxin, and snake antivenom.