Immunity - Session 4

Active immunity occurs when the body is exposed to a pathogen and produces its own immune response. This type of immunity is generally long lasting and sometimes lifelong. This is because the immune system retains a memory of the pathogen. This allows for a faster and more efficient response if the pathogen is encountered again.

When a pathogen enters the body, it is recognized by the immune system. Can you tell how a pathogen is recognized by the immune system? The pathogen carries antigens on its surface. These antigens are recognized as foreign by the immune system.

Cells such as dendritic cells, macrophages and B-cells capture the pathogen and process its antigens. These antigen presenting cells then produce the antigens on their surface using molecules called Major Histocompatibility Complex proteins.Helper T-cells recognize these antigens presented on Major Histocompatibility Complex class two molecules by antigen presenting cells. When a helper T-cellc recognizes the antigen MHC complex, it becomes activated.

Activated helper T-cells release cytokines, which are signaling molecules that stimulate other immune cells, including B-cells and cytotoxic T-cells. B-cells directly bind to the antigen through their B-cell receptors. They can also internalize and produce the antigen on MHC class two molecules to helper T-cells. Helper T-cells assist in fully activating B-cells by providing additional signals through cytokines and cell to cell interactions.

Activated B-cells proliferate and differentiate into plasma cells. Plasma cells produce large quantities of specific antibodies and memory B-cells. Antibodies neutralize pathogens. They label them for destruction by the macrophages.

Infected cells produce pathogen derived antigens on MHC class one molecules. Cytotoxic T-cells recognize these antigens. Cytotoxic T-cells need activation signals from both the infected cells and cytokines released by helper T-cells. Once activated, cytotoxic T-cells release perforins and granzymes. This leads to the apoptosis which is the programmed cell death of infected cells.

We know that the memory B-cells and memory T-cells were produced by the plasma cells. Memory B-cells remain in the body for years. They can quickly respond by producing antibodies if the same pathogen is encountered again. Memory helper T-cells and memory cytotoxic T-cells provide rapid responses to future infections by the same pathogen. This ensures a faster and more effective immune response.

Active immunity can be acquired in two ways. When a person is exposed to an organism that causes diseases, his immune system responds by producing specific antibodies and memory cells. This is called natural active immunity. For example, if someone recovers from chickenpox, they typically gain lifelong immunity against the virus. This is because their immune system has developed a memory of it.

Active immunity can also be acquired artificially. Artificial active immunity is acquired through vaccination. Vaccination is a medical procedure that involves the administration of a vaccine to stimulate an individual's immune system to recognize and fight specific pathogens.

A vaccine contains antigens from a pathogen, such as weakened or inactivated forms of the pathogen, or pieces of it like proteins or genetic material. When these antigens are introduced into the body, they do not cause the disease but instead stimulate the immune system to produce a response. The immune system responds to the vaccine by producing antibodies and memory cells. For example, the measles vaccine contains a weakened form of the virus. It stimulates the immune system to protect against future infections.

Passive immunity involves the transfering of antibodies from one individual to another individual. This provides immediate but temporary protection against specific infections. Unlike active immunity, where the immune system generates its own response, passive immunity supplies ready made antibodies. It offers quick defense without the need for the immune system to recognize and respond to the pathogen itself.

There are two primary forms of passive immunity. These are natural passive immunity and artificial passive immunity. Natural passive immunity occurs when antibodies are passed from mother to baby. During pregnancy, maternal antibodies cross the placenta and provide the fetus with immune protection. After birth, antibodies are also transferred through breast milk. It gives the newborn temporary immunity against infections the mother has encountered.

In artificial passive immunity, the antibodies are administered from an immune individual to a non immune individual. These antibodies can come from human or animal sources and are given through injections. For example, antivenoms for snake bites, rabies immunoglobulin, or monoclonal antibodies for diseases like COVID-19 are forms of artificial passive immunity.

In passive immunity, antibodies are introduced into the body either naturally or artificially. These antibodies circulate in the bloodstream and are immediately available to bind to and neutralize pathogens. The antibodies bind to specific antigens on the pathogen, neutralizing it and preventing it from infecting cells or spreading. Antibodies also label pathogens for destruction.

Since the recipient’s immune system does not produce these antibodies, there is no formation of memory cells. Once the antibodies degrade and are removed from the body, immunity diminishes. The protection provided by passive immunity is short term. It usually lasts a few weeks to a few months, depending on the half life of the antibodies.