What is Adaptive Immunity?

Adaptive immunity, also known as acquired immunity or specific immunity, is a subsystem of the overall immune system that provides a tailored defense against specific pathogens. Unlike the innate immune system which offers broad protection, the adaptive immune system is highly specialized to fight each particular pathogen the body encounters.

Key features of adaptive immunity include:

  • Specificity: Adaptive immune responses are custom-made to target specific antigens (foreign substances that induce an immune response)
  • Memory: The adaptive immune system can “remember” past infections and mount a stronger, quicker response if the same pathogen is encountered again
  • Diversity: Through genetic recombination, the adaptive immune system can generate a vast number of different antigen receptors to recognize a wide variety of pathogens

Components of the Adaptive Immune System

The main players in adaptive immunity are white blood cells called lymphocytes. The two major types are:

B ell
  1. B cells (B lymphocytes): Originate and mature in the bone marrow. When activated, they differentiate into plasma cells that secrete antibodies.
  2. T cells (T lymphocytes): Originate in the bone marrow but mature in the thymus. There are several types including helper T cells that activate other immune cells, and cytotoxic T cells that directly kill infected or cancerous cells.

Both B and T cells have antigen-specific receptors on their surface. Each cell is specific for a particular antigen, ensuring the response is targeted.

Humoral vs Cell-Mediated Immunity

Adaptive immunity includes both humoral and cell-mediated responses:

  • Humoral immunity: Mediated by antibodies produced by B cells. Antibodies circulate in blood and lymph, binding to specific antigens to neutralize them or mark them for destruction.
  • Cell-mediated immunity: Mediated by T cells. Cytotoxic T cells directly destroy infected or abnormal cells, while helper T cells release cytokines to recruit and activate other immune cells.

Immunological Memory

A key feature of adaptive immunity is immunological memory. After the initial exposure to an antigen, memory B and T cells are formed. These cells can survive for years, allowing the immune system to “remember” the pathogen and mount a faster, stronger response upon re-exposure. This is the basis for long-lasting immunity after an infection or vaccination.

Activation of Adaptive Immunity

Adaptive immune responses are triggered when pathogens evade or overcome innate immune defenses. Antigen-presenting cells like dendritic cells and macrophages display pieces of the pathogen to lymphocytes, along with co-stimulatory signals to activate them. This process, along with cytokines produced during the innate immune response, stimulates the proliferation and differentiation of antigen-specific B and T cells to fight the infection.

the adaptive immune system provides powerful, specific, and long-lasting protection against pathogens. Its ability to generate a vast repertoire of lymphocytes, each with a unique antigen receptor, allows it to recognize virtually any pathogen. Immunological memory then provides enhanced protection if that pathogen is encountered again. Together, the innate and adaptive immune systems work to defend the body against the wide range of infectious threats it faces.

Innate Immunity vs. Adaptive Immunity: A summary

AttributeInnate ImmunityAdaptive Immunity
Response TimeFast: minutes or hoursSlow: days
SpecificityOnly specific for molecules and molecular patterns associated with general pathogens or foreign particlesHighly specific! Can discriminate between pathogen vs. non-pathogen structures, and miniscule differences in molecular structures
Major Cell TypesMacrophages, Neutrophils, Natural Killer Cells, Dendritic Cells, Basophils, EosinophilsT cells, B cells, and other antigen presenting cells
Key ComponentsAntimicrobial peptides and proteins, such as toxic granulesAntibodies
Self vs. Nonself DiscriminationInnate immunity is based on self vs. nonself discrimination, so it has to be perfectNot as good as the innate immune system, but still pretty good at determining which is which. Problems in self vs. nonself discrimination result in autoimmune diseases
Immunological MemoryNoneMemory used can lead to faster response to recurrent or subsequent infections
Diversity and CustomizationLimited: Receptors used are standard and only recognize antigen patterns. No new receptors are made to adapt the immune response