Antibody-Mediated Immunity

An intruding antigen is sensed by the B-cells in the lymph nodes.  The B-cells become sensitised and transform into a larger secretory type of cell called the plasma cell.  The plasma cell then proliferates, forming a clone, and all the cells in the clone synthesise a specific protein molecule called the antibody, which is secreted into the plasma.  Upon encountering the antigen, the antibodies bind with the antigen molecules and deactivate them.  This whole process takes from days to weeks.

Classes of Immunoglobulins (Antibodies)

IgG Most abundant antibody; found in blood, lymph and intestines; protects against viruses and bacteria.
IgA Found mainly in sweat, tears, saliva, mucus, milk and GI secretions; provides localised protection on mucous membranes against bacteria and viruses.
IgM First antibody class to be secreted by plasma cells after initial exposure to an antigen; found in blood, lymph and as antigen receptors on surface of B cells.
IgD Found in blood, lymph and as antigen receptors on surface of B cells; involved in activation of B cells.


Least abundant of all antibodies; found on mast cells and basophils; involved in allergic and hypersensitivity reactions and provides protection against parasitic worms.

Memory Cells

After the antigens are deactivated, the antibodies usually diminish in number.  However, upon second exposure to the same antigen, the body's antibody production is more rapid and more intense.  This enhanced response is due to a particular type of plasma cell called the memory cell.  B-cells produce memory cells upon their first exposure to the antigen.  Memory cells learn how to produce the antibody at first, then they rest until the second exposure to the same antigen, when they become activated rapidly and form numerous clones to produce large amounts of the antibody.

Immunological Memory

Immunological Memory (1)


The memory cells are involved in immunisation by vaccination.  Here, the body is intentionally exposed to a small amount of dead or transformed antigen in order to sensitise the immune system and form memory cells.  When the body is exposed to the same antigen later, the immune system will be ready to defend against it and antibody production will be quick and intense.