fetus and in the primary and secondary immune responses

During development, any lymphocytes bearing receptors that bind to molecules in the fetus are eliminated. Thus the only lymphocytes that persist at birth are those that failed to bind to anything, that is, those that have never encountered an antigen. Once the immune system has matured (early in infancy), primary immune responses develop in response to never-before encountered antigens, and secondary immune responses develop with subsequent exposures to the same antigens. Innate immunity is, at all times, independent of prior antigen exposure.