The humoral repsonse removes the pathogenic microorganisms which don't enter the cells and requires antibodies produced by B lymphocyctes. Different B lymphocyte cells produce different antibodies, each of which is complementary to (i.e. fits the shape of, like jigsaw pieces) a signle type of antigen (present on the surface of pathogens). The B lymphocyte cell is activated once its antibody has bound to an antigen and it replicates itself to produce many clones, all producing the same antibody. The large amount of antibodies can then target and lead to the break down of the pathogen. This repsonse pathway leads to the formation of memory cells.
The cell-mediated repsonse is required when the pathogenic microorganism has entered the body cells and does not require antobodies. Once the pathogen has entered the cells it is much harder to remove and so the infected cell has to be destroyed. This is done by macrophages engulfing the invading pathogen, and presenting its foreign antigen on its surface (this is why macrophages are called antigen presenting cells). T helper cells recognise the antigen and activate cytotoxic T cells (or T killer cells) which then kill the infected cell.