An antigen is any material to which the immune system reacts. Antigens are usually foreign materials from invading micro-organisms but can include foods and inorganic chemicals. Individual or specific antigens are tiny, of molecular size. Sometimes the body will react to its own molecules in a harmful way, but the immune system has a way of eliminating most cells reacting in this way. This is called auto-immunity. There are literally millions upon millions of potential different antigens the body might have to deal with in a lifetime.

Antibodies are a group of proteins made by B-cells. Each antibody is specific for a particular antigen. The best way of imagining how an antibody is specific for an antigen is to think of the three-dimensional fit between a lock and key. On the surface of a B-cell there are the receptors for a specific antigen. Most importantly, B-cells usually cannot perform this function without help from CD4 type-2 (Th2) helper cells. A B-cell is activated when its antigen receptor recognises its specific antigen, causing release of specific antibody into the bloodstream as the B-cell makes copies of itself.

T-cells also have molecular receptors on their surface that recognise specific antigens. These are structurally related to antibodies. Again, the best analogy for the way T-cell receptors recognise specific antigen is to think of a lock and key. CD4 and CD8 molecules on the surface of T-cells help the T-cell receptor attach to an antigen when it is presented to them.