Immunity
The body's multi-layered defence system against pathogens and foreign substances.
Immunity is the biological defence mechanism that protects an organism from disease by identifying and destroying pathogens and tumour cells. It is broadly divided into two interconnected systems: non-specific (innate) immunity and specific (adaptive) immunity.
### Non-Specific (Innate) Immunity
This is the body's first and immediate line of defence. It is non-specific because it acts against all pathogens in the same way. A key process in non-specific immunity is phagocytosis, carried out by specialised white blood cells called phagocytes (e.g., macrophages and neutrophils).
The process of phagocytosis involves several steps:
### Specific (Adaptive) Immunity
This is a highly specific defence system that targets particular pathogens and provides long-lasting protection. It involves two main types of white blood cells called lymphocytes: T-lymphocytes (T-cells) and B-lymphocytes (B-cells).
1. Cell-Mediated Immunity (involving T-cells)
T-cells mature in the thymus gland. They are responsible for destroying infected body cells.
* Helper T-cells (TH cells): These cells have receptors that are complementary to the antigens presented by APCs. When a TH cell binds to an APC, it becomes activated.
* Activated TH cells release chemical messengers called cytokines. Cytokines have several roles, including stimulating the division and differentiation of B-cells and activating other T-cells.
* Cytotoxic T-cells (TC cells): Also known as killer T-cells, these are activated by cytokines. They seek out and destroy body cells that are infected with the pathogen. They do this by releasing a protein called perforin, which creates pores in the infected cell's membrane, causing water to enter and the cell to undergo lysis (bursting).
2. Humoral Immunity (involving B-cells and Antibodies)
B-cells mature in the bone marrow and are responsible for producing antibodies.
* Clonal Selection: Each B-cell has a specific antibody-like receptor on its surface. When an antigen (either free or on an APC) with a complementary shape binds to this receptor, and the B-cell is stimulated by cytokines from a TH cell, that specific B-cell is selected for activation.
* Clonal Expansion: The selected B-cell divides rapidly by mitosis, producing a large number of genetically identical cells (a clone).
* Differentiation: This clone of B-cells differentiates into two cell types:
* Plasma Cells: These are short-lived, specialised cells that synthesise and secrete vast quantities of specific antibodies into the blood and lymph. Antibodies bind to antigens on pathogens, neutralizing them or marking them for destruction by phagocytes (a process called opsonisation).
* Memory Cells: These are long-lived cells that remain in circulation. They provide immunological memory. If the same pathogen invades again, these memory cells recognise it and mount a rapid and powerful response.
### Principles of Vaccination
Vaccination creates immunity without the individual having to suffer from the disease. It introduces a safe form of an antigen—such as a dead, weakened (attenuated), or component part of a pathogen—into the body. This stimulates a primary immune response.
* Primary Response: The initial exposure to the antigen (via the vaccine) triggers the specific immune response. B-cells and T-cells are activated, leading to the production of plasma cells, antibodies, and crucially, memory cells. This response is relatively slow and produces a lower concentration of antibodies.
* Secondary Response: If the individual is later exposed to the live pathogen, the memory cells facilitate a secondary immune response. This response is much faster, stronger, and longer-lasting. A large number of plasma cells are produced quickly, secreting a high concentration of antibodies that destroy the pathogen before symptoms can develop.
This process is known as artificial active immunity. Vaccination of a large proportion of the population leads to herd immunity, which protects vulnerable, unvaccinated individuals by reducing the pathogen's circulation.
Key Points to Remember
- 1The immune system has non-specific (phagocytosis) and specific (lymphocyte-mediated) responses.
- 2Phagocytes engulf pathogens and present their antigens, becoming Antigen-Presenting Cells (APCs).
- 3Helper T-cells are activated by APCs and release cytokines, which coordinate the specific immune response.
- 4B-lymphocytes undergo clonal selection and differentiate into antibody-producing plasma cells and long-lived memory cells.
- 5Cytotoxic T-cells directly destroy infected host cells by releasing perforin, which causes cell lysis.
- 6Immunological memory, provided by memory cells, results in a faster and stronger secondary immune response.
- 7Vaccination induces artificial active immunity by stimulating a primary immune response and creating memory cells.
- 8Antibodies produced by plasma cells neutralise pathogens by binding to their specific antigens.
Pakistan Example
Polio Eradication Initiative in Pakistan
Pakistan's nationwide polio vaccination campaigns are a direct application of immunological principles. The Oral Polio Vaccine (OPV) used contains a live, attenuated (weakened) poliovirus. When administered to a child, the virus's antigens trigger a primary immune response. Helper T-cells activate B-cells, leading to the production of plasma cells that secrete antibodies against the poliovirus, and crucially, the formation of B- and T-memory cells. This confers long-term, artificial active immunity. If the child is later exposed to the wild poliovirus, the memory cells will mount a rapid and powerful secondary response, preventing the virus from causing paralysis. The extensive campaigns aim to achieve 'herd immunity', where a high percentage of the population is immune, thereby protecting the few who are unvaccinated and breaking the chains of transmission in the community.
Quick Revision Infographic
Biology — Quick Revision
Immunity
Key Concepts
Polio Eradication Initiative in Pakistan
Pakistan's nationwide polio vaccination campaigns are a direct application of immunological principles. The Oral Polio Vaccine (OPV) used contains a live, attenuated (weakened) poliovirus. When administered to a child, the virus's antigens trigger a primary immune response. Helper T-cells activate B-cells, leading to the production of plasma cells that secrete antibodies against the poliovirus, and crucially, the formation of B- and T-memory cells. This confers long-term, artificial active immunity. If the child is later exposed to the wild poliovirus, the memory cells will mount a rapid and powerful secondary response, preventing the virus from causing paralysis. The extensive campaigns aim to achieve 'herd immunity', where a high percentage of the population is immune, thereby protecting the few who are unvaccinated and breaking the chains of transmission in the community.