ABSTRACT
Slightly higher than normal numbers of apparently mature T- and B-lymphocyte populations (as well as NK cells) are present in the blood of newborn individuals. Even so, the ability to mount an immune response to certain antigens may be lacking at birth. Thus, children under 2 years do not usually make antibody to the polysaccharides of pneumococcus or H. influenzae. In general, the ability to respond to a specific antigen depends on the age at which the individual is exposed to the antigen. There are a variety of explanations for this sequential appearance of specific immunity, including (i) sequential expression of genes encoding receptors for each antigen; (ii) immaturity of some B or helper T-cell populations or of antigen-presenting cells (e.g., macrophages and dendritic cells); (iii) passive maternal antibody that binds antigen and removes it, thereby interfering with the development of active immunity.
