The higher vertebrate immune system is extremely efficient in fighting foreign intruders of many shapes and forms. This exclusive property requires the ability to recognize an almost infinite number of different antigens and to distinguish foreign from self. The defence against microbial pathogens is operated through two distinct cellular processes: the innate immune system that is already present in invertebrates (such as the Drosophila fly) and the adaptive immune response, a more sophisticated and complex response which appeared during evolution of vertebrates.
Innate immunity is inborn and provides an all-purpose defence against invasion. The innate immune system includes cells, called macrophages, able to find and engulf micro-organisms, while others release chemicals that kill the organism directly. Other cells, such as dendritic cells, are able to recruit specialized immune cells to the region of infection and play thus an important role in signalling the presence of intruders to constituents of the adaptive immune system. Cells of the innate immune system express a family of conserved receptors (pattern recognition receptors) which react to a wide range of pathogen-derived substances, including molecules found in the cell walls of many kinds of bacteria. Adaptive or acquired immunity is mediated by lymphocytes, a class of cells characterized by the ability to generate an enormous array of antigen-specific receptors (immunoglobulins or antibodies for B lymphocytes and T cell receptors for T lymphocytes) by recombination of gene segments. Unlike receptors expressed by cells of the innate immune system, antigen-specific receptors such as antibodies are produced in almost infinite variety and match a particular pathogen like a key in a lock.
The adaptive immune response stems from dynamic interactions between cells of the innate response (antigen processing cells such as dendritic cells), T lymphocytes and B lymphocytes. Dendritic cells are endowed with the capacity to engulf antigen and to display antigenic fragments on their membrane histocompatibility molecules. Under appropriate conditions (the recognition of an infectious event by cells of the innate immune system), dendritic cells display antigenic fragments to naïve helper T cells and promote their proliferation and activation. These cells can in turn activate cytotoxic T lymphocytes and B lymphocytes to pathogen recognition and destruction. Communication between these different classes of cells occurs during brief cellular contacts (immune synapses) occurring in peripheral lymphoid organs (spleen and lymph nodes) draining the site of infection. Cells of the immune system exchange information through locally secreted cytokines or membrane-bound receptors.