Adaptive Immunity

Adaptive immunity generates receptor diversity before it knows what it will meet. RAG-mediated V(D)J recombination shuffles gene segments and adds junctional nucleotides, producing a repertoire whose theoretical diversity vastly exceeds the number of genes in the genome. Antigen then does not instruct the response; it selects the rare pre-existing clone that already fits, which proliferates. T cells recognise only peptide fragments displayed on MHC, whereas B cells bind native three-dimensional structure directly.

Speed is the price of specificity. A primary response takes roughly five to seven days before antibody is detectable, while a secondary response is faster and yields higher-affinity IgG through germinal-centre affinity maturation and class switching. Durability varies by antigen and adjuvant rather than by any general property of the system: one dose of yellow fever 17D gives neutralising antibody lasting decades, tetanus toxoid needs boosting roughly every ten years, and vaccinia-specific T cells remain detectable half a century after smallpox vaccination.

Because adaptive responses are clonal and slow, nothing adaptive contributes to the first hours of an infection. Repeated exposure to a foreign protein or peptide is exactly the schedule that generates memory, which is why anti-drug antibodies accumulate over months of therapy rather than appearing at first dose.

The claim to distrust is generic boosting. There is no dial that raises adaptive immunity in the abstract, because every response is specific to an antigen, and a raised lymphocyte count or serum immunoglobulin level says nothing about whether a useful clone expanded. Thymic peptide products sold on this basis have not demonstrated antigen-specific responses in controlled trials.