B Cells

B cells (aka B lymphocytes) produce antibody when exposed to their complementary antigen. These antibodies can cause engulfment of infectious bacteria, neutralization of virions and induction of the complement cascade.

In the bone marrow, B cells complete their hematopoietic differentiation from stem cells into IgM⁺,IgD^weak immature virgin B cells. Next, in the medulla, B cells fully activate to become IgM⁺,IgD⁺ mature virgin B cells. Upon exposure to antigens, B cells in the medulla begin producing antibodies that flow through the lymph to the entire body. This helps create the body’s immune memory, with large amounts of antibody loaded in the bone marrow and at germinal centers. B cell activation occurs in the following steps:

1. An antigen flows through an afferent lymphatic vessel and into a node’s cortical sinus.
2. The antigen percolates through the node until getting trapped by reticular cells and dendritic cells.
3. Macrophages within the node trap and present the antigen. If it is a foreign antigen, either or both virgin B cells or memory B cells may react.

The B cells discussed so far are conventional B cells (aka B-2 cells). There is another subset of B cells known as B-1 cells (aka CD5 B cells, since some species’ B-1 cells display CD5). B-1 cells arise from stem cells during fetal life and self-renew via division of existing cells. While conventional B cells usually produce IgG, B-1 cells typically produce IgM and have little or no IgD. CD5 B cells secrete antibodies in response to TI-2 polysaccharides, leading to complement and removal of bacteria. This occurs within 48 hours of antigen exposure, as a bridge until the adaptive T cell response can activate. Unlike the T cell response, the B-1 response does not have memory. The table below (adapted from Immunology, 6^th edition) further compares and contrasts conventional (aka B-2) and B-1 cells.