Oct 13, 2020
INNATE AND ADAPTIVE IMMUNITY
There are two subsystems within the immune system, known as the innate (non-specific) immune system and the adaptive (specific) immune system. Both of these subsystems are closely linked and work together whenever a germ or harmful substance triggers an immune response.
“The immune system can be simplistically viewed as having two “lines of defense”: innate immunity and adaptive immunity. Innate immunity represents the first line of defense to an intruding pathogen. It is an antigen-independent (non-specific) defense mechanism that is used by the host immediately or within hours of encountering an antigen. The innate immune response has no immunologic memory and, therefore, it is unable to recognize or “memorize” the same pathogen should the body be exposed to it in the future.
Adaptive immunity, on the other hand, is antigen-dependent and antigen-specific and, therefore, involves a lag time between exposure to the antigen and maximal response. The hallmark of adaptive immunity is the capacity for memory which enables the host to mount a more rapid and efficient immune response upon subsequent exposure to the antigen. Innate and adaptive immunity are not mutually exclusive mechanisms of host defense, but rather are complementary” 
« The immune system is an extraordinarily complex operation, employing numerous different types of cells capable of recognizing harmful foreign substances, organizing a response, carrying out attacks and ultimately guarding against future invasions. The coordination required for such a system to work is gargantuan in scope and attests to a high degree of evolutionary sophistication. »
Ralph Steinman discovered, in 1973, a new cell type that he called the dendritic cell. In cell culture experiments he demonstrated that dendritic cells can activate T-cells, a cell type that has a key role in adaptive immunity and develops an immunologic memory against many different substances.
Despite initial skepticism about the uniqueness of the dendritic cells, Steinman kept saying and proving that the existence of dendritic cells justified the link between the occurrence of an infection in peripheral tissues and clonal expansion/selection observed in lymphoid organs, thus opening therapeutic prospects for prophylactic and therapeutic vaccination (against cancer and microbes), as well as for inducing tolerance (for the treatment of allergy, transplantation and autoimmunity). Dendritic cells, defined as antigens-presenting cells, are the main T-cell stimulating cells insuring the cross-communication between innate and adaptive immunity.
For this discovery, Dr. Steinman received the 2011 Nobel Prize in Physiology or Medicine.
IMMUNE MEMORY & IMMUNISATION
A unique property of the immune system and another important characteristic of adaptive immunity is immune memory.
“Immune memory is defined as a changed status of the immune system of a host after an acute infection (or vaccination), leading to a more effective response against reinfection.” 
It means that the immune system can remember the antigens that previously activated it and respond with greater vigor upon re-encounter with the same antigen a second time.
Figure 2.10. Diagram Showing the Phenomenon of Immune Memory.
(A) When transplanting the same graft to the identical donor for a second time, the rejection time is much shorter than the first time. (B) When immunizing the body with the same antigens for the second time, the resulting antibody response is more intense than the first time, whereas the antibody response to different antigens produced at the second immunization is no different from that at the first time.
Adapted and modified from Goldsby et al. (2002).
Although immune memory was thought for a long time to be the exclusive property of the adaptive immune response, studies have led to the hypothesis that the innate immune system also exhibits adaptive characteristics, a property named ‘trained immunity’. A better understanding of trained immunity will result in a better understanding of host defence mechanisms and the pathogenesis of immune-mediated diseases.
Immunologic memory provides the rationale for the development of vaccines. “Immunization is the process whereby a person is made immune or resistant to an infectious disease, typically by the administration of a vaccine. Vaccines stimulate the body’s own immune system to protect the person against subsequent infection or disease.” 
 Ralph M. Steinman, Laurence Zitvogel, The fabulous legacy of a Nobel Prize Laureate.