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Adaptive Immunity: B and T Cells, Antibodies, and Immune Memory

The acquired immune system is one of the two main immunity strategies found in vertebrates (the other being the innate immune system). Like the innate system, the adaptive immune system includes both humoral immunity components and cell-mediated immunity components and destroys invading pathogens. Unlike the innate immune system, which is pre-programmed to react to common broad categories of pathogen, the adaptive immune system is highly specific to each particular pathogen the body has encountered. Adaptive immunity creates immunological memory after an initial response to a specific pathogen, and leads to an enhanced response to future encounters with that pathogen.

Figures (10)

Each antibody binds to a specific antigen in a highly specific interaction analogous to a lock and key.
Schematic structure of an antibody: two heavy chains (blue, yellow) and the two light chains (green, pink). One of the antigen binding sites is circled.
Some antibodies form complexes that bind to multiple antigen molecules.
Antibodies (A) and pathogens (B) free roam in the blood.The antibodies bind to pathogens, and can do so in different formations such as:opsonization,neutralisation, andagglutination.A phagocyte (C) approaches the pathogen, and the Fc region (D) of the antibody binds to one of the Fc receptors (E) of the phagocyte.Phagocytosis occurs as the pathogen is ingested.
Clonal expansion and monoclonal versus polyclonal proliferation
Model of an antibody showing beta strands
Surface model of an antibody at the molecular level
The secreted mammalian IgM has five Ig units. Each Ig unit (labeled 1) has two epitope binding Fab regions, so IgM is capable of binding up to 10 epitopes.
The complementarity determining regions of the heavy chain are shown in red (PDB: 1IGT​)
Simplified overview of V(D)J recombination of immunoglobulin heavy chains

Key Points

  • B cells and T cells are derived from the same multipotent hematopoietic stem cells, and look identical to one another until after they are activated.
  • T cells are useless without antigen-presenting cells to activate them, and B cells are disabled without T cell help.
  • Antibodies are a critical part of the adaptive immune system.
  • Antibodies (also known as immunoglobulin, Ig), are large Y-shaped proteins used by the immune system to identify and neutralize foreign objects.
  • IgG can also form hexamers, though no J chain is required.
  • The most ancient Ig class, IgM, is membrane-bound and then secreted upon stimulation of cartilaginous fish B cells.

Terms

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