Secondary immune response: A secondary immune response is triggered by a second injection of antigens or a secondary immunization. The secondary response begins sooner after the antigen injection, is stronger and of a greater affinity than the primary immune response. It is borne mainly by IgG antibodies.

Selection: A cell is selected by an antigen when its receptors recognize and bind said antigen. If the cell then reproduces and forms a clone, we speak of clonal selection; if it is killed by the antigen binding, we speak of negative selection or clonal deletion.

Self-tolerance: One speaks of self-tolerance or own tolerance when the immune system does not react to endogenous antigens.

Sensitization refers to the immunization before an allergic reaction, with the same antigen which later triggers the acute immune response. Allergic reactions occur only in sensitized people.

Serum is the liquid component of coagulated blood.

Somatic recombination: Complete exons, which code the V region of every antibody or T cell receptor chain, result through somatic recombination of the individual gene segments for immune receptors during the lymphocyte maturation. This process occurs only in somatic cells, and the changes are accordingly not inherited.

Specific defence: A part of the immune defence, based on the selective recognition of foreign antigens.

Spleen: The spleen is a primary lymphatic organ. It consists in part of a red pulp which is involved in the elimination of old blood cells and a white pulp with lymphatic cells which react to antigens entering the spleen with the bloodstream.

Stem cells are undifferentiated cells without a specific function from which all other tissue types form. The various blood and immune cells also form from stem cells.

Suppressor T cells are T cells which can suppress the activities of naïve T cells or effector T cells when mixed with them. We do not know exactly what properties the suppressor T cells have. We also do not know how they recognize antigens and how they become active.