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When learning about the immune response, my teacher mentioned that all the bodies B cells are present at birth, and there is one to counter every disease. But if this is the case, why should the primary response take a long time?

As far as I know, the primary response involves the non-specific response followed by the specific response, where APCs cause the activation of T cells resulting in clonal selection and expansion of T and B cells. The B cells then differentiate into plasma cells and produce antibodies. It results in the memory cells being produced, which can quickly reengage the specific response the next time the pathogen enters.

Why doesn't the body just make memory cells to begin with? As if it has already encountered every possible pathogen. I presume it is just a lack of specificity in the A level teaching, but I am curious to know what I am missing here.

Thanks, Jamie

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up vote 9 down vote accepted

These so called virgin B-cells (becase they never had contact with their antigen) which are present in your body only have random B-cell receptors (when secreted, these are the antibodies), mostly of a relatively low binding affinity. These B-cells undergo division, re-arrangement and somatic hypermutation of the B-cell receptor and clonal selection. Only cells producing high-affinity antibodies survive this selection process and build later the pool for the memory cells and the plasma cells. It is like in the image below (from here):

enter image description here

Using this approach we have a big pool of low affinity antibodies (which somehow fit). These are then used as the basis for the generation of high affinity (perfectly fitting) antibodies, which target only the pathogen. And the antibodies are available relatively fast.

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Ah, that explains it perfectly! –  J_mie6 Apr 14 '14 at 16:56

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