B1 cell
B1 cells are a sub-class of B cell lymphocytes that are involved in the humoral immune response. They are not part of the adaptive immune system, as they have no memory, but otherwise, B1 cells perform many of the same roles as other B cells: making antibodies against antigens and acting as antigen-presenting cells. B-1 cells are classified as innate-like lymphocytes due to their T cell independent activation. They express high levels of Toll-like receptors (TLRs), TLR4 in particular, which recognizes lipopolysaccharide (LPS) from Gram-negative bacteria. B-1 cell TLR binding of LPS results in B-1 cell division and differentiation into IgM-secreting plasma cells without TH cell help. These B1 cells are commonly found in peripheral sites, but less commonly found in the blood. These cells are involved in antibody response during an infection or vaccination. B-1 cells also respond to cytokine signals from other innate immune cells, like mast cells. Receptors for IL-5 promote B-1 cell survival and antibody secretion. This allows coordination of B-1 response with broader inflammatory events at sites of infection.
There are two types of B1 cells subsets, B1a cells and B1b cells. B1b cells have been shown to be capable of memory responses. B1b cells also can recognize protective antigens in bacteria, which is unique because they are targeting something internal.