The humoral immune response is one of two main arms of the immune system. In this response, the immune system triggers specific B cells to proliferate and secrete large amounts of their specific antibodies. These antibodies can then combat a particular microorganism or virus and thereby stop an infection.
The humoral immune response has an activation phase and effector phase. During the activation phase, helper T (TH) cells become activated against a particular antigen. In the effector phase, activated TH cells trigger specific B cells to proliferate and release antibodies. These antibodies then bind to the invader and fight the infection.
The phagosome fuses with a lysosome, which contains digestive enzymes. The enzymes break down the engulfed particle into fragments, in a phenomenon called antigen processing.
Within the cell, the processed antigens combine with class II MHC proteins. The complex is displayed on the macrophage's membrane. This display is known as antigen presentation, and macrophages are considered antigen-presenting cells.
A helper T cell participates in the next stage of the humoral immune response. This helper T cell has T-cell receptors that can bind to both the class II MHC protein and this particular presented antigen. This binding triggers the macrophage to release the cytokine interleukin-1, which activates the helper T cell.
The activated helper T cell now releases its own cytokines, which stimulate the cell to proliferate. The cell proliferates to form a clone of helper T cells, all with the same T-cell receptors. These receptors are specific for the antigenic determinant of the original processed antigen.
The effector phase begins with a B cell. This B cell has membrane-bound IgM receptors that are specific for the same antigen as originally engulfed by the macrophage. An IgM receptor binds to the antigen, and the cell engulfs the complex by receptor-mediated endocytosis.
The internalized vesicle fuses with a lysosome, which contains digestive enzymes. The enzymes digest the antigen, processing it into fragments. The processed antigen is then attached to class II MHC molecules and displayed on the surface of the B cell.
A helper T cell from the clone of cells in the activation phase can now bind to the antigen displayed by the B cell. The T-cell receptor specifically recognizes the antigen on the class II MHC protein. Upon binding, the helper T cell releases cytokines that stimulate the B cell to divide and create a clone of identical cells.
The resulting B cells develop into either long-lived memory cells or into antibody-secreting plasma cells. Plasma cells have extensive endoplasmic reticulum and numerous ribosomes.
Plasma cells are essentially antibody factories. They produce and secrete antibodies of the specificity identical to that of the surface receptors on the parent B cell. Like the surface IgM receptors on the parent B cell, the antibodies secreted by the plasma cells can bind to and inactivate the original antigen.
By the end of the humoral response, the immune system has activated specific B cells, which produce and release large amounts of their specific antibodies. Of the millions of different B cells produced by the immune system, only those that can recognize the invader are activated. This specificity prevents the body from making all types of antibodies possible (which would very likely harm the body, in addition to being energetically costly).
Antibodies defend the body in a number of ways. For example, if the antigen is a toxin or a virus, the binding of antibodies to the antigens isolates the antigens, preventing them from contacting and harming cells of the body. Additionally, antigens that are coated with antibodies are easily recognized by macrophages, engulfed, and digested. Antibodies also stimulate the complement system, which consists of a group of proteins that can poke holes in the cell walls of bacteria.