The innate immune system is naturally present in you since birth and protects you from pathogens regardless of experiences. Your immune system adapts itself every time when it comes across a new disease and naturally develops pathogen-specific immunity, known as adaptive immunity. Adaptive immunity comes into existence only after an infection or immunization and hence is 'acquired' during life.

Passive Immunity Vs. Active Immunity
  • Adaptive immunity is subdivided into two major types, naturally acquired immunity and artificially acquired immunity. Naturally acquired immunity is the immunity that is acquired through contact with pathogen, but the contact is not deliberate. Artificially acquired immunity is the immunity that is purposefully developed through deliberate actions such as vaccination.
  • Both, naturally acquired and artificially acquired immunities are further subdivided into active and passive immunity, depending on whether immunity is induced in the host or passively transferred from a immune host.
  • Immunity, that is named as active, is generated in the host by the antigen itself. It lasts much longer, sometimes life-long. Antigen is a substance like a toxin or enzyme which stimulates the immune response of the body by especially enhancing the production of antibodies by the immune system.
  • You can develop passive immunity by acquiring antibodies or activated T-cells from an immune host. Your body "downloads" critical information about a wide range of pathogens in a managed way and uses it for defense mechanism! Passive immunity does not last longer - usually it lasts for a few months only.
  • The basic adaptive immunity is also classified as humoral immunity and cell mediated immunity, depending upon the cells involved. Humoral immunity is said to be active when you generate your own antibodies and passive when you receive antibodies from other individuals. Similarly, cell mediated immunity is active when your own T-cells are stimulated and passive when T-cells come from another human being.
More About Active Immunity

This immunity is concerned with both the cell-mediated and humoral aspects of immunity. It also requires input from the innate immune system. When a pathogen activates B-cells and T-cells, the memory B-cells and T- cells are built up in your body. Throughout your lifetime, these memory cells will 'recall' each pathogen encountered and will be ready to give a strong fight if the body comes across with the pathogen again. With such active and adaptive immunity, the body's immune system keeps itself ready for future challenges.

Naturally Acquired
Naturally acquired active immunity comes into existence when you are exposed to a live pathogen (a disease producing agent) so that your body develops a primary immune response, leading to immunological memory. The type of the immunity is called 'natural' because it is not generated by deliberate exposure. The formation of active immunity can be severely disturbed due to immune system dysfunction.

Artificially Acquired
Artificially acquired active immunity is the immunity developed through vaccination. Vaccine contains an antigen and induces a primary response against that antigen. Vaccine can not cause symptoms of the disease. Louis Pasteur is known as the pioneer of vaccination. Pasteur developed a method of treating the infectious agents for the infectious diseases so that they wouldn't be able to cause serious diseases.

Human beings have been fascinated by the concept of immunity since thousands of years. In Europe, the concept of active immunity developed in an attempt to control smallpox. However, immunization did exist in various forms for at least a thousand years. It is believed that around 1000 A.D, people knew about immunization. Because of the studies and researches done by the scientists, right since 17th and 18th century, we are able to lead a healthy and happy life now.