In order to understand the immunological role of B cells one must first introduce the immune system.
The immune system is often regarded as an entity consisting of an organised network of different layers that work together and interact in order to protect the host. These different components communicate with the external environment and with each other in order to initiate an immune response against potentially harmful pathogens such as viruses, bacteria, fungi, parasites and toxins. This highly advanced system attempts to protect the host from constituents that are recognised as foreign whilst preventing reactions to self and to other non-harmful external elements. Primarily, the immune system has been divided into the innate and the adaptive, which differ with regard to reaction times and the ability to generate memory.
The innate immune system is considered the host’s first line of defence against pathogens, thus facilitating a rapid immune response within minutes to hours of infection. Components of innate immunity include (i) epithelial barriers between the environment and the host, comprising of skin and mucosal surfaces of the gastrointestinal and respiratory tract; (ii) phagocytes including monocytes, macrophages and neutrophils and other immune cells such as dendritic cells, NK cells, basophils and eosinophils; and (iii) members of the complement system, anti-microbial peptides and cytokines.
The adaptive immune system becomes activated when a pathogen manages to penetrate the innate immune system of the host. Although this adaptive response takes a longer time frame in order to reach its full-scale potential (days to weeks), it remains the most specialised out of the two systems with increased specificity to the encountered pathogen alongside ability to generate B and T cell memory that assists in mounting a rapid immune response upon recurrent infection with the previously encountered pathogens. The most important cells of the adaptive immune system are the B and T lymphocytes, where the B cells originate mainly in the bone marrow while T cell development starts in the thymus, hence the acronyms “B” and “T”.
Cellular components of innate and adaptive immunity. Innate immunity has low specificity and affinity, but reacts within hours of an infection by the activation of phagocytic cells (macrophages and neutrophils) and natural killer (NK) cells through the recognition of common conserved bacterial and viral components. It therefore represents the first line of defence against microbes. The adaptive immune response develops later upon the activation of T and B lymphocytes by the dendritic cells through antigen presentation on MHC. This process requires more time yet results in a specific immune response, where the dendritic cells act as mediators between the innate and the adaptive immune systems. Moreover, the generation of T and B cell memory during adaptive immunity can aid in mounting a rapid immune response upon recurrent infection. Figure was produced using Servier Medical Art and inspired by Abbas et al.