Active Immunity Definition, Characteristics, Types, Examples

Active immunity is a type of immunity that develops when the body’s immune system responds to the presence of a foreign substance, such as a pathogen (like a virus or bacteria) or a vaccine. It involves the production of specific immune responses, including the activation of immune cells and the production of antibodies, which provide protection against future encounters with the same substance.

Types of Active immunity:

1. Naturally Acquired Active Immunity:

This type of immunity occurs when a person’s immune system is exposed to a live pathogen, contracts the disease, and then mounts an immune response. After recovery, the immune system “remembers” the pathogen, which provides immunity against future infections with the same pathogen. This is the natural way that individuals develop immunity to many infectious diseases.

2. Artificially Acquired Active Immunity:

This type of immunity is induced intentionally through medical interventions. It occurs when a person is exposed to a weakened or inactivated form of a pathogen (such as a vaccine) or specific components of a pathogen (like proteins or polysaccharides). The immune system responds by producing an immune response without causing the actual disease. This results in the development of immunity against the pathogen.

Active Immunity Characteristics

1. Specificity:

Active immunity targets specific pathogens or antigens. The immune response is tailored to recognize and combat particular foreign substances, such as viruses, bacteria, or other pathogens.

2. Memory:

After exposure to a specific antigen, active immunity leads to the development of immunological memory. This means that the immune system “remembers” the antigen, allowing for a rapid and heightened response upon subsequent exposure to the same pathogen.

3. Long–lasting Protection:

Active immunity provides long-lasting protection against the specific pathogen to which the immune system has been exposed. This protection can persist for years, and in some cases, it may last a lifetime.

4. Involves Immune Responses:

Active immunity requires the activation of various components of the immune system, including immune cells (like T cells and B cells) and the production of antibodies. These responses work together to eliminate or neutralize the invading pathogen.

5. Natural or Induced:

Active immunity can be naturally acquired through exposure to a live pathogen, where the individual contracts the disease and mounts an immune response. Alternatively, it can be artificially induced through vaccination, where a person is exposed to a weakened or inactivated form of the pathogen or its components.

6. Adaptive and Specific:

Active immunity is an adaptive immune response, meaning it adapts and evolves to effectively combat specific pathogens. It develops over time in response to exposure and exhibits specificity for particular antigens.

7. Primary and Secondary Responses:

The first exposure to a specific antigen triggers a primary immune response. This response may take time to fully develop and may not be as robust. However, it leads to the production of memory cells, which provide the basis for a stronger and faster secondary immune response upon re-exposure to the same antigen.

8. Can be Transferred:

Active immunity can be transferred naturally from a mother to her infant through breast milk, where the infant receives antibodies produced by the mother’s immune system. Additionally, it can be artificially induced through the administration of specific antibodies (known as passive immunization) for immediate but temporary protection.

9. Vaccination:

One of the primary methods of inducing active immunity is through vaccination. Vaccines contain weakened or inactivated forms of pathogens or their components, which stimulate the immune system to produce a protective immune response without causing the actual disease.

Mediators of Active immunity

1. Antigens:
   • Antigens are foreign substances, typically proteins or polysaccharides, that stimulate an immune response. They are recognized by the immune system as “non-self” and trigger the production of specific immune responses.
2. Antigen–Presenting Cells (APCs):
   • APCs are specialized cells (such as dendritic cells, macrophages, and B cells) that capture, process, and present antigens to T cells. They play a crucial role in initiating and regulating the immune response.
3. T Lymphocytes (T Cells):
   • T cells are a type of white blood cell that play a central role in cell-mediated immunity. They are responsible for recognizing specific antigens presented by APCs and coordinating the immune response. There are several types of T cells, including helper T cells, cytotoxic T cells, and regulatory T cells.
4. B Lymphocytes (B Cells):
   • B cells are another type of white blood cell that are responsible for humoral immunity. When activated by antigens, B cells differentiate into plasma cells, which produce and secrete antibodies. These antibodies circulate in the blood and body fluids, binding to and neutralizing specific antigens.
5. Antibodies (Immunoglobulins):
   • Antibodies are proteins produced by plasma cells in response to antigen exposure. They have specific binding sites that recognize and bind to antigens, marking them for destruction or neutralization by other components of the immune system.
6. Memory Cells:
   • Both T cells and B cells have memory cell populations. These cells “remember” specific antigens they have encountered before. Upon re-exposure to the same antigen, memory cells mount a faster, more robust immune response, providing long-lasting protection.
7. Cytokines:
   • Cytokines are small proteins secreted by various immune cells that act as signaling molecules. They play a crucial role in coordinating and regulating immune responses. Cytokines can influence the activation, differentiation, and proliferation of immune cells.
8. Complement System:
   • The complement system consists of a group of proteins that work together to enhance the immune response. It can help in the destruction of pathogens, promote inflammation, and assist in the clearance of immune complexes.
9. Major Histocompatibility Complex (MHC):
   • MHC molecules are proteins found on the surface of cells that present antigens to T cells. They play a critical role in the activation of T cells and the initiation of cellular immune responses.
10. Interferons:
    • Interferons are a group of cytokines that are released by cells in response to viral infections. They have antiviral properties and help regulate immune responses to viral pathogens.
11. Effector Cells:
    • Effector cells are the specialized cells that directly carry out the immune response, such as cytotoxic T cells that destroy infected cells, and plasma cells that produce antibodies.

Active Immunity Examples

Naturally Acquired Active Immunity:

1. Chickenpox (Varicella):
   • When an individual contracts the varicella-zoster virus (VZV) and recovers from chickenpox, they develop active immunity against future infections with the same virus. This means that the person is less likely to get chickenpox again.
2. Measles:
   • After a person recovers from measles, they develop active immunity against the measles virus. This provides long-lasting protection, making reinfection unlikely.
3. Mumps:
   • Recovering from a mumps infection leads to the development of active immunity against the mumps virus. This immunity reduces the likelihood of getting mumps again.
4. Rubella (German Measles):
   • After recovering from rubella, a person develops active immunity against the rubella virus. This protects against future rubella infections.

Artificially Acquired Active Immunity (Vaccination):

1. Polio (Poliovirus):
   • The oral polio vaccine (OPV) or inactivated polio vaccine (IPV) contain weakened or inactivated forms of the poliovirus. When administered, the vaccine stimulates the immune system to produce an immune response, leading to the development of active immunity against polio.
2. Hepatitis B:
   • The hepatitis B vaccine contains a portion of the hepatitis B virus surface protein. Upon vaccination, the immune system recognizes this protein as foreign and mounts an immune response. This leads to the development of active immunity against hepatitis B.
3. Influenza (Flu):
   • Seasonal influenza vaccines are formulated each year to protect against specific strains of the influenza virus. The vaccine contains inactivated or weakened forms of the virus. Vaccination induces an immune response, providing active immunity against the targeted strains.
4. Tetanus:
   • The tetanus vaccine contains a toxoid (inactivated toxin) produced by the bacterium Clostridium tetani. When vaccinated, the immune system responds by producing antibodies against the toxin, providing active immunity against tetanus.
5. Diphtheria:
   • The diphtheria vaccine contains inactivated diphtheria toxin produced by the bacterium Corynebacterium diphtheriae. Immunization induces an immune response, leading to the development of active immunity against diphtheria.
6. COVID-19 (SARS-CoV-2):
   • COVID-19 vaccines, such as those developed by Pfizer-BioNTech, Moderna, and others, contain a portion of the spike protein of the SARS-CoV-2 virus. Upon vaccination, the immune system recognizes this protein and mounts an immune response, leading to the development of active immunity against COVID-19.

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