The term 'humoral immunity' refers to any soluble factor found in the blood or lymph that contributes to host protection. In reality most of the time, humoral immunity is describing the role of antibodies, specialized molecules which can attach to antigens. However, there are several other important molecules that are part of this response that are worthy of note, especially the acute phase proteins, a group of plasma proteins, many of which are produced by the liver, whose concentrations increase several-fold during an inflammatory response. Specialized molecules, called interleukins and other cytokines act in many ways like hormones, to stimulate growth and proliferation of these cells in immune organs such as the thymus and bone marrow.
B-cells are found in follicles of lymphoid tissues. Though not as numerous as T-cells (B-cells only 10% to 20% of all lymphocytes) B-cells have a critical function: They are the antibody manufacturers. When an antigen is presented by the T-cell to a specific B-cell, it enlarges (blast transformation) and rapidly reproduces , producing clones of identical daughter lymphocytes or plasma cells. Because of this rapid rise in the number of essentially identical cells, B cells can produce large amounts of a specific antibody to the antigen. In order to appreciate what makes a B cell so special, let�s take a look at what they make.
To understand what an antibody is, we must first understand the significance of the word 'antigen,' which is easy, since basically anything that causes an immune response is called an antigen. An antigen may be harmless, such as grass pollen, or harmful, such as the flu virus. Disease-causing antigens are called pathogens. The immune system is designed to protect the body from pathogens. Thus, anything that induces a subsequent immune response, as evidenced by the activation and proliferation of B and/or T lymphocytes, is referred to as an antigen.
The important thing to remember about antigens is that they induce antibodies, specialized molecules manufactured by the immune system to tag and destroy anything the immune system considers 'non-self.' Antibody either destroys the antigen directly, or coats it so that phagocytic cells (neutrophils and monocytes) can ingest and destroy the particle or invader. Virtually all proteins are anitgenic. Carbohydrates (usually polysaccharides) are potentially very immunogenic. Polysaccharides that are part of complex molecules (like cell surface Glycoproteins) elicit an immune response directed specifically against the Polysaccharide? moiety. An example are the blood type antigens, which derive their immunologic specificity from the polysaccharides on the surface of the red blood cells and intestinal lining.