How is a B lymphocyte activated?
The antigen in its native form reacts directly with the surface immunoglobulins of B lymphocytes, without any intervention of antigen presenting cells or T lymphocytes. Later, the antigen is ingested, fragmented, associated to the SLA II and then it is expressed in the membrane or the lymphocyte. Thus, it works as the rest of the APCs, with two main differences: a) Antigens can be recognized in their native form by surface immunoglobulins and b) Phagocytosis do not occur. Once the antigen is expressed in the membrane of B lymphocytes (and if there is an appropriate stimulus from a CD4+ lymphocyte), the production of immunoglobulins will start. This cooperation is essential for the initiation of antibody production against the majority of antigens.The collaboration between B and CD4+ lymphocytes takes place when both cells come in contact, thanks to the SLA II and the antigen, and to the liberation of lymphokines, especially interleukin-4.
