The B cells encounter the antigen directly
When an immune response is mounted against a foreign antigen, the B cells of the immune system produce large amounts of antibodies (Ab). There are two ways to make a B cell make Ab. One way is by direct activation of B cells through the process called Antigen Induced Proliferation (AIP).
Another way to activate B cells is through the process called Antibody Induced Cell Activation (AICA). In AIP, the antigens cross-link the B cell receptors, c-Jun protein related products which activates the B cells to proliferate and produce more Ab. In AICA, the B cells encounter the antigen directly without cross linking the BCR.
Antigens are a complex molecule made up of peptides, carbohydrates, and proteins. They are usually formed by organisms that have a different genetic make-up. Bacteria, viruses, fungi, parasites, and yeast are common sources of antigens. They can be found everywhere in the environment. In human beings, antigens are found in bacteria, viruses, and parasites.
Antigens may be denatured, modified, or degraded during passage through the digestive tract. Thus, antigenic peptides are usually short fragments of a larger molecule, but even small fragments can stimulate the immune system to produce Abs.