The adaptive immune response is called “adaptive” because over the course of an infection, it adjusts to optimally handle the particular microorganism responsible for the disease. Adaptive immune responses are more sophisticated than innate immune responses in that it is highly specific to the pathogen involved. Adaptive immune responses also have the capacity to recall past events. Thus if one is immunized with an antigen, an immune response (either antibody or cell mediated) appears after several days, rises rapidly and then gradually declines in what is referred to as a primary immune response. Upon encountering this same antigen later in life, however, the response is much faster due to memory cells which quickly proliferate when they encounter the antigen in what is sometimes called a secondary immune response.

Adaptive immune responses are carried out by white blood cells (leukocytes) called lymphocytes. The human adaptive immune response can be divided into 2 distinct branches: (1) antibody or humoral responses which targets extracellular pathogens by stimulating B cells to produce antibodies, and (2) cellular immune responses which targets intracellular pathogens by activating CD8+ cytotoxic T cells and macrophages. CD4+ helper T cells are crucial for regulating both types of responses.

Using products of the RAG1 and RAG2 genes, B and T lymphocytes somatically rearrange the V, D, and J elements of their immunoglobulin (Ig) and T cell receptor (TCR) genes to create as many as 1011 different clones of B and T lymphocytes that express distinct antigen receptors. The receptors on B lymphocytes recognize conformations of native antigen, which may be protein, carboyhydrate, or simple chemical groups, whereas the receptors of most T lymphocytes recognize only peptides, which are derived from protein antigens, that are bound to cell surface proteins termed major histocompatibility complex (MHC) class I and class II.

Humoral Response

The humoral branch of adaptive immunity is uniquely equipped to eliminate extracellular pathogens. It is characterized by the production of large numbers of antibodies specific for epitopes on the foreign pathogen.

Cell-Mediated Response

If antibodies were the only agents of immunity, pathogens which managed to evade them and colonize the intracellular environment would escape the immune system. The cell mediated branch of adaptive immunity is well equipped to eliminate intracellular pathogens. Specific cells include CD8+ cytotoxic T lymphocytes (Tc cells or CTLs) and cytokine-secreting CD4+ TH cells. Nonspecific cells include NK cells and nonlymphoid cell types such as macrophages, neutrophils and eosinophils.

Chemotactic peptides generated by the activation of complement in response to antigen-antibody complexes can also contribute to assembling the cell types required for a cell mediated response.

A number of cells (such NK cells, macrohpages, monocytes, neutrophils and eosinophils) that have cytotoxic potential express membrane receptors for the Fc region of the antibody molecule. When antibody is specifically bound to a target cell, these receptor bearing cells can bind to the antibody Fc region and cause lysis of the target cell in what is referred to as antibody-dependent cell-mediated cytotoxicity (ADCC).

The induction of an effective cellular immune response against tumors also depends on a local increase of inflammatory cytokines in the tumor mircroenvironment. This fact serves as the basis for much research currently going on in cancer immunotherapy. One approach to do this is to use tumor cells genetically engineered ex vivo to produce various cytokines, such as IL-12, which are important in inducing a cellular immune response which destroys tumor cells.