Primary and Secondary Immune Responses

1° Immune Response

  1. Following the first exposure to a foreign antigen, a lag phase occurs in which no antibody is produced, but activated B cells are differentiating into plasma cells. The lag phase can be as short as 2-3 days, but often is longer, sometimes as long as weeks or months.
  2. The amount of antibody produced is usually relatively low.
  3. Over time, antibody level declines to the point where it may be undetectable.
  4. The first antibody produced is manily IgM (although small amounts of IgG are usually also produced).

2° Immune Response

  1. If a second dose of the same antigen is given days or even years later, an accelerated 2° or anamnestic immune response (IR) occurs. This lag phase is usually very short (e.g. 3 or 4 days) due to the presence of memory cells.
  2. The amount of antibody produced rises to a high level.
  3. Antibody level tends to remain high for longer.
  4. The main type of antibody produced is IgG (although small amounts of IgM are sometimes produced).

Note: The crossmatch attempts to prevent a 2° immune response by detecting any antibody present, and then ensuring that only antigen-negative red cells are transfused. It cannot prevent a 1° immune response because only autologous red cells or red cells from an identical twin will introduce no foreign antigens into a person being transfused.

In blood banking, a 1° immune response doesn't always cause mainly IgM antibody to be produced. Sometimes only IgG antibody can be detected (e.g., for antibodies in the Duffy or Kidd systems). Similarly, a 2° immune response does not always cause mainly IgG antibody to be produced. Sometimes, only IgM antibody is produced (e.g., for antibodies in the MN or Lewis systems).

Primary and Secondary Immune Responses