THE CONTEXT
The passage describes initial removal of particulate antigens and compares the timing, cells, and antibody characteristics of primary and secondary responses.
Non-immune removal and antigen localisation
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During the first, non-immune phase, phagocytic cells engulf antigens, which are broken down and eliminated.
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Reticuloendothelial cells break down the antigens, which are excreted in urine; about 70–80 per cent is eliminated within one or two days.
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Subcutaneously introduced antigens are mainly localised in draining lymph nodes, with only small amounts found in the spleen.
Response timing and B cells
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The primary response usually has a lag period of 4–10 days and reaches its peak in 7–10 days; it involves naive B cells.
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The secondary response usually has a lag period of 1–3 days and reaches its peak in 3–5 days; it involves memory B cells.
Antibody characteristics
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The primary peak antibody response depends on the antigen. The secondary peak response is usually 100–1000 times greater than the primary response.
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IgM predominates early in the primary response, whereas IgG predominates in the secondary response.
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Antibody affinity is lower in the primary response and higher in the secondary response.
Antigens and immunisation requirements
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Primary responses occur to both thymus-dependent and thymus-independent antigens; secondary responses occur only to thymus-dependent antigens.
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Primary immunisation requires a comparatively high antigen dose, favourably with adjuvants for protein antigens. Adjuvants are not required for the secondary response.