B cells circulate throughout our blood, lymph, and secondary lymphoid organs.
Sometimes, in the lymph nodes and spleen, soluble components from a pathogen appear. B cells, expressing a molecule called a B cell receptor at its surface, can bind those components (called antigens) directly.
Cross-linking occurs, wherein many nearby B cell receptors recognize components of the same antigen, and intracellular signaling occurs within the B cell.
Sometimes, the B cell doesn’t need any further assistance. If the pathogen-derived components it has recognized are highly repetitive and have resulted in lots of cross-linking, the B cell’s signal may be strong enough to lead to antibody production. This process is known as T cell-independent B cell activation.
The bulk of B cell activation occurs in a T cell-dependent manner, however. Intracellular signaling after antigenic engagement helps the B cell to start looking a lot like a dendritic cell. It begins to express the MHC molecule at its surface, as well as costimulatory molecules like B7 and CD40.
The B cell internalizes the antigen it was carrying with its B cell receptor, and the antigen goes on to be broken down and presented on a MHC molecule. This is ideal for a T cell to recognize it.
After recognizing the presented peptides, a molecule on the T cell (CD40L) interacts with a molecule on the B cell (CD40). This “help” is enough for the B cell to begin producing and releasing antibodies, but the T cell can be of aid in one other way.
Depending upon the T cell’s synthesized cytokines, there is the potential to influence the types of antibodies that the B cell makes.
Known as “class switching”, when instructed to do so by B cell cytokines, B cell genes encoding for antibodies can have some of their DNA cut out, resulting in the production of antibodies of a different shape and function. In this way, depending on the cytokines provided by the T cell, 5 different antibody types may be produced during different infections, allowing for the optimization of the immune response.
Some antibodies, like IgM, IgG, and IgA, function to neutralize the pathogen. Pathogens need to interact with a host cell in order to enter and cause disease. Antibodies can get in the way, binding the pathogen so as to prevent this interaction with host cells.
Some antibodies, like IgG, function to opsonize pathogens. They can bind a pathogen and make it easier for cells like macrophages to undergo phagocytosis.
This article’s content was created based upon the background accrued over the course of the author’s Honours BSc Degree in Microbiology & Immunology, which included Immunology, Virology, and Bacterial Pathogenesis course-work and exposure to topics from Janeway’s Immunobiology.
