Macrophages eliminate bacteria by phagocytosis. After degradation of the bacterium, the phagocyte presents fragments of the pathogen via major histocompatibility complex (MHC) class II molecules upon its surface (Peppelenbosch 2000) and binds a T cell. Subsequently both cells are activated and release a variety of mediators during this process. Among the released cytokines, interleukin-2 (IL-2) is set free and binds to corresponding receptor upon the surface of a specific B cell (Roitt 1994).

This is one of the relevant signals for the B cell to proliferate and finally differentiate into a plasma cell. The plasma cell synthesizes and releases specific immunoglobulins that serve as opsonins and lead to neutralization and elimination of the antigens that have induces this process.

After trauma and during sepsis significant immunosuppression may occur. Several mediators, e.g. prostaglandine E2 and Interleukin-10, contribute to this phenomenon (Sfeir 2001).
Immunosuppression may be partly due to a disturbed antigen presenting process with a consecutive lack of specific immunoglobulins.