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Primary versus Secondary Response

To understand the history of HDFN due to anti-D, it is useful to review the immune response.

A primary (1°) immune response is the response that occurs following the first exposure to a foreign antigen.

A secondary (2°)/anamnestic immune response occurs following subsequent exposures.

The main differentiating features as related to producing anti-D during pregnancy are shown in the table and figure.

1o immune response 2o immune response
1. Following the first exposure to the D antigen, a lag phase occurs in which no anti-D is produced, but activated B cells differentiate into plasma cells. The lag phase can be as short as several days, but often is longer. 1. When exposure to D occurs in subsequent pregnancies, the lag phase is short (3–7 days) due to the presence of memory B cells that quickly differentiate into antibody-secreting plasma cells.
2. Depending on the antibody detection method, it often takes 5–15 weeks before anti-D is detectable in serologic tests. 2. An increase in anti-D is usually detectable within days.
3. The amount of anti-D produced is relatively low. 3. The amount of anti-D rises to a higher level.
4. Anti-D titers decline fairly rapidly and may become undetectable. 4. Anti-D titers tend to remain higher for longer but eventually decline.
5. The first anti-D produced is mainly IgM (although small amounts of IgG are usually also produced). 5. The main type of anti-D produced is IgG (although small amounts of IgM may be produced).