Active Immunity- Definition, Characteristics, Types, Examples

Active immunity is a type of immunity that results from the production of antibodies by the immune system in response to the presence of an antigen. An antigen is a foreign substance that can trigger an immune reaction, such as a virus, bacterium, or toxin. Active immunity can be acquired through natural or artificial means.

Active immunity has the following characteristics:

• It involves the production of antibodies and memory cells by the individual`s own immune system. Antibodies are proteins that bind to specific antigens and mark them for destruction by other immune cells. Memory cells are long-lived cells that remember the antigen and can quickly mount a stronger response upon re-exposure.
• It requires time to develop. The first exposure to a pathogen or its antigen triggers a primary immune response, which takes days or weeks to reach its peak. However, if the same pathogen or antigen is encountered again, the memory cells initiate a secondary immune response, which is faster and more effective than the primary response.
• It is long-lasting and can provide lifelong protection. The memory cells produced during active immunity can persist for years or even decades in the body, ready to respond to any future exposure to the same pathogen or antigen. This is why some diseases, such as chickenpox or measles, usually occur only once in a lifetime.
• It can have side effects. Active immunity can sometimes cause adverse reactions, such as fever, inflammation, pain, or allergic responses. These are usually mild and temporary, but in rare cases they can be severe or even life-threatening. Moreover, active immunity can also lead to autoimmune diseases, in which the immune system mistakenly attacks the body`s own tissues.

Active immunity can be classified into two types: natural active immunity and artificial active immunity. Both types involve exposure to an antigen and the subsequent production of antibodies and memory cells by the immune system. However, the source and mode of exposure differ in each type.

Natural active immunity

Natural active immunity is the immunity that is acquired when an individual is exposed to a live pathogen, either clinically or subclinically, through natural infection. This means that the individual develops the disease and symptoms, and then recovers from it. As a result of the infection, the immune system generates a primary response against the pathogen and produces memory cells that can recognize and eliminate the same pathogen in the future. Natural active immunity is usually long-lasting and can confer lifelong protection against some diseases.

For example, when a person gets infected with chickenpox virus (varicella-zoster virus), they develop a rash and fever for a few days or weeks. After recovering from the infection, they become immune to chickenpox for the rest of their life. This is because their immune system has learned to recognize and destroy the chickenpox virus if it ever encounters it again.

Artificial active immunity

Artificial active immunity is the immunity that is acquired when an individual is exposed to an antigen that is derived from a pathogen, but does not cause the disease. This type of exposure is deliberate and induced by immunization or vaccination. Vaccination involves introducing a weakened, killed, or inactivated form of a pathogen or its components into the body to stimulate the immune system to produce antibodies and memory cells against it. Artificial active immunity also takes time to develop, but it can provide long-term or lifelong protection against some diseases.

For example, when a person receives a vaccine against measles virus (measles-mumps-rubella vaccine), they are exposed to a weakened form of the virus that does not cause measles. However, their immune system recognizes the virus as foreign and produces antibodies and memory cells against it. If they ever encounter the wild-type measles virus in the future, their immune system will be able to mount a faster and stronger secondary response and prevent them from getting sick.

Artificial active immunity has many advantages over natural active immunity. It can prevent diseases that are serious, contagious, or have no effective treatment. It can also protect people who are at high risk of infection or complications due to age, health conditions, or occupation. It can also reduce the spread of diseases in the population and contribute to herd immunity. However, artificial active immunity also has some limitations and challenges. It may not be effective against all strains or variants of a pathogen. It may require multiple doses or boosters to maintain adequate levels of protection. It may also cause adverse reactions or side effects in some individuals.

Active immunity is mediated by two types of immune responses: humoral immunity and cell-mediated immunity. These responses involve different types of cells and molecules that work together to eliminate or kill various pathogens.

Humoral immunity

Humoral immunity refers to the immunity that is mediated by antibodies, which are proteins produced by a type of white blood cell called B cells. Antibodies are found in the blood and the mucosal secretions, such as saliva and tears. They can recognize and bind to specific antigens, which are molecules that are part of a pathogen or a foreign substance. By binding to antigens, antibodies can neutralize their infectivity and mark them for elimination by other immune cells or mechanisms. For example, antibodies can activate the complement system, which is a cascade of proteins that can lyse (break down) the membrane of bacteria or viruses. Antibodies can also bind to the surface of pathogens and facilitate their uptake by phagocytes, which are cells that can engulf and digest microbes. Alternatively, antibodies can bind to toxins or venoms and prevent them from harming the host cells.

Humoral immunity is the main defense mechanism against extracellular microbes, which are microbes that live outside the host cells, such as bacteria, fungi, and parasites. Humoral immunity can also protect against some viruses that are present in the blood or secretions.

Cell-mediated immunity

Cell-mediated immunity refers to the immunity that is mediated by T cells, which are another type of white blood cell. T cells have receptors on their surface that can recognize antigens that are presented by other cells, such as antigen-presenting cells (APCs). APCs include B cells, dendritic cells, and macrophages, which are specialized phagocytes that can process and display antigens on their surface. There are different types of T cells that have different functions in the immune system:

• Helper T cells (TH cells): These cells secrete various cytokines, which are signaling molecules that can activate or regulate other immune cells or processes. For example, TH cells can stimulate B cells to produce antibodies, or enhance the killing ability of phagocytes or natural killer (NK) cells. TH cells can also help to activate another type of T cell called cytotoxic T cells (CTLs).
• Cytotoxic T cells (CTLs): These cells can directly kill infected or abnormal host cells by releasing molecules that induce apoptosis (programmed cell death) or by forming pores in the cell membrane. CTLs are especially important for eliminating intracellular microbes, which are microbes that live inside the host cells, such as viruses, some bacteria, and some parasites. CTLs can also kill tumor cells or transplanted cells that are recognized as foreign by the immune system.
• Memory T cells: These cells are long-lived and can remember a specific antigen that they have encountered before. They can quickly respond to a subsequent exposure to the same antigen by proliferating and differentiating into effector T cells (TH cells or CTLs). Memory T cells provide long-term protection against reinfection by the same pathogen.
• Regulatory T cells (Treg cells): These cells suppress or modulate the activity of other immune cells to prevent excessive inflammation or tissue damage. They also help to maintain self-tolerance, which is the ability of the immune system to distinguish between self and non-self antigens and avoid attacking the host`s own tissues.

Cell-mediated immunity is essential for defending against intracellular microbes, as well as some extracellular microbes that can resist phagocytosis or antibody-mediated neutralization. Cell-mediated immunity can also protect against tumors or transplanted organs.

Active immunity and passive immunity are two types of immunity that can protect us from infections and diseases. However, they differ in how they are acquired, how long they last, and how they work. Here are some of the main differences between active and passive immunity:

• Source of antibodies: Active immunity results when our own immune system produces antibodies to fight a pathogen or its antigen. Passive immunity results when we receive antibodies from another source, such as a mother, an animal, or a vaccine.
• Exposure to pathogen: Active immunity requires exposure to a live or weakened form of a pathogen, either through natural infection or vaccination. Passive immunity does not require exposure to a pathogen, but only to its antibodies.
• Onset of protection: Active immunity takes time to develop, as the immune system needs to recognize the antigen and produce specific antibodies. Passive immunity provides immediate protection, as the antibodies are already present and ready to act.
• Duration of protection: Active immunity is long-lasting, and sometimes lifelong, as the immune system retains memory cells that can quickly respond to the same antigen in the future. Passive immunity is short-lived, and usually lasts for a few weeks or months, as the antibodies are gradually degraded or eliminated by the body.
• Examples: Some examples of active immunity are natural immunity from recovering from an infection (such as measles or chickenpox), or vaccine-induced immunity from receiving a vaccine (such as polio or tetanus). Some examples of passive immunity are maternal immunity from receiving antibodies through the placenta or breast milk, or artificial immunity from receiving antibody-containing blood products (such as immune globulin or monoclonal antibodies).

Active and passive immunity are both important for our health and well-being. They can work together to enhance our immune response and prevent disease. For example, newborn babies receive passive immunity from their mothers, which protects them until they develop their own active immunity through natural exposure or vaccination. Similarly, people who are exposed to a new or emerging pathogen may benefit from passive immunity through antibody therapy, which can reduce the severity of the infection and buy time for their active immunity to kick in.

I hope this helps you understand the difference between active and passive immunity. If you have any questions or feedback, please let me know. 😊

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