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A Practical Guide to Immunisation
Figure 3.1: Immune Response following exposure to antigen.
The primary immune response develops in the weeks following exposure to an antigen and is
mainly IgM antibody. It is made by cells in lymph nodes, spleen and bone marrow and circulates in
the blood and lymphatic system. IgM antibodies activate the complement system. IgM antibodies
have a short half-life (5 days).
The secondary immune response is faster and more powerful and is predominantly IgG antibody.
It is made in lymph nodes, spleen and bone marrow. The IgG antibody is smaller than IgM and its
half life is about 3 weeks. It also passes through the placenta from mother to baby giving the baby
antibodies to which the mother is immune.
Active immunity can be divided into antibody and cell mediated responses.
3.4.1 Antibody mediated immunity
Antibody mediated responses are produced by B lymphocytes (B cells) and plasma cells. When a
B cell encounters an antigen that it recognises, the B cell is stimulated to proliferate and produce
large numbers of lymphocytes, secreting antibody to the antigen. Replication and differentiation
of B cells into plasma cells is regulated by contact with the antigen and by interactions with T cells,
macrophages and complement. The antibodies produce immunity through neutralising toxins,
blocking adhesion and cell entry by organisms, neutralising and preventing viral replication or
complement-mediated killing.
3.4.2 Cell mediated immunity
Cell mediated immunity is controlled by a subset of lymphocytes called T lymphocytes or T cells.
T cells stimulate B cells to produce antibodies and play an inhibitory role and control the level and
quality of the immune response. Cytotoxic T cells recognise and destroy infected cells and activate
phagocytes to destroy pathogens that they have taken up.
3.5 How do vaccines work?
Vaccines work by inducing active immunity and by providing immunological memory.
Immunological memory allows the immune system to recognise and respond rapidly to natural
infection when exposed at a later stage and to prevent or modify the effect of that disease.
Antibodies can be detected in blood or serum but, even if undetectable, memory usually still exists.
Vaccines are made from inactivated (killed) or attenuated live organisms, secreted products,
recombinant components or components of cell walls. Vaccines must contain sufficient antigenic
mass to stimulate a desired response. The components of vaccines are listed in Table 3.1.
Chapter 3: Immunity and how vaccines work Page 23