To understand the history of HDFN due to anti-D, it is useful to review the immune response.
A primary (1°) immune response 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 Table 1 and graphed in the image.
Table 1. Primary vs. Secondary Immune Response.| Primary Immune Response | Secondary Immune Response |
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.
| 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. |
Depending on the antibody detection method, it often takes 5–15 weeks before anti-D is detectable in serologic tests.
| An increase in anti-D is usually detectable within days.
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The amount of anti-D produced is relatively low.
| The amount of anti-D rises to a higher level.
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Anti-D titers decline fairly rapidly and may become undetectable.
| Anti-D titers tend to remain higher for longer but eventually decline. |
The first anti-D produced is mainly IgM (although small amounts of IgG are also usually produced).
| The main type of anti-D produced is IgG (although small amounts of IgM may be produced).
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