2 Immunological memory

An important feature of the adaptive immune response is immunological memory – on subsequent encounters with the same antigen, the immune response is faster and more effective. This effect can be seen in the primary and secondary antibody responses against an antigen, as shown in Figure 7. Note: titre is a measure of how much antibody is present in the serum, and in this case it is shown on a logarithmic scale.

Figure 7 Characteristics of primary and secondary antibody responses

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This figure is a line graph. The horizontal axis is labelled days and is marked from zero to 42 at intervals of 7 days. The vertical axis is labelled log antibody titre and is marked from zero to 100 000 at tenfold intervals. The primary antigen challenge, at time zero, and the secondary antigen challenge, at 30 days, are both marked. There are two plots, showing the change in IgM and IgG antibody titre respectively, during both the primary and secondary response. In the primary response phase, there is a lag time of 4 days, after which IgM titre begins to rise; it peaks at 10 units by 8 days, and then declines to near zero by 14 days. The IgG titre begins to rise later than IgM, around 7 days after the challenge; it peaks later too, at 13 days, and reaches a higher titre, of about 50 units. IgG titre then falls to below one unit within 20 days after the primary challenge. In the secondary response phase, there is no lag time for either class of antibody. By 38 days (i.e. 8 days after the challenge), IgM titre had risen to 8 units, after which it began to decline. In contrast, IgG titre reaches several thousand units by day 38, and at 42 days (i.e. 12 days after secondary challenge) has not declined from this level.

Figure 7 Characteristics of primary and secondary antibody responses

Notice four key differences between the primary and secondary antibody response. On the secondary response:

• The lag time before the appearance of antibodies is shorter.
• The peak titre is much higher.
• IgG antibodies predominate; these antibodies also bind more strongly to the antigen.
• The high levels of IgG antibodies are maintained for longer.

There are three main explanations for the improved secondary immune response:

• The number of lymphocytes that can respond to the antigen increases by clonal division during the primary response.
• Some of the responding lymphocytes have undergone differentiation and maturation steps, which means they can react more swiftly and/or to lower levels of antigen stimulation; these cells act as ‘memory cells’.
• Memory cells are seeded into lymphoid organs and mucosal tissues around the body, so they are on-site to respond as soon as the antigen or pathogen is encountered again.

These findings provide one of the rationales for vaccination. If a person has been vaccinated with a harmless variant of an antigen or pathogen and made a primary immune response against it, they are then able to mount a secondary immune response if they subsequently encounter the real pathogen.