Several times I've read the statement "a child's immune system isn't fully developed until two years old, so you should wait until then to vaccinate." This is, to be blunt, a gross oversimplification. On this page, I'll dig into immune system development a bit more. No doubt this is still oversimplified, but perhaps a bit less so than the one-sentence sound bite.
As you'd guess from the use of the word "system", there are lots of things involved in the body's immunity to disease. Major components include:
These cells develop from "pluripotential hematopoietic stem cells" starting at a gestational age of about 5 weeks. They are distributed through various organs in the lymphatic system as the fetus develops. "T and B lymphocytes are the only components of the immune system that have antigen-specific recognition capabilities; they are responsible for adaptive immunity." In other words, it's the T and B cells that are important in the sort of immunity that vaccination promotes. T cells show up in these organs at the indicated gestational age:
The lymphatic system consisting of these organs is responsible for delivering immune system components to the various parts of the body.
Immunization requires a complex dance of many factors. I don't understand everything I've read, and I'm pretty sure researchers don't understand the whole story yet. But here's the thumbnail version.
The point of injecting an antigen (a substance capable of provoking an immune response) is to set off the primary antibody response. Steps in the primary antibody response include:
When the antigen shows up in the body again -- or another protein that binds to the same FDC sites - it provokes a secondary antibody response. This is what we think of as immunity:
The antibodies produced by the secondary antibody response bind to the antigen proteins that triggered the response. These proteins are typically on the surface of the invading organism. The other end of the antibodies bind to NK cells, which are always present. The NK cells then kill the target cell, which prevents it from multiplying and causing disease.
Normal infants have the capability to develop T-cell and B-cell responses to antigens at birth.
Infants also start life with some immunoglobulin antibodies acquired from the mother. These antibodies cross the placental barrier, but not all types are transmitted equally. In particular, infants start with antibodies to viruses and gram-positive organisms, but not to gram-negative organisms. The "gram" here is not a unit of weight, but the name of a stain that distinguishes broad classes of bacteria. Gram-negative organisms are responsible for many diseases: gonorrhea, pertussis, salmonella, shigella, cholera, and many more. Escherichia coli (e. coli) is another common gram-negative organism.
Immunoglobulins are divided into five classes, designated IgA, IgD, IgE, IgG, and IgM. The capacity of the body to produce each of these varies with age. IgG and IgM are the antibodies that protect against infectious diseases:
"However, normal infants cannot produce antibodies to polysaccharide antigens until usually after age 2 yr unless the polysaccharide is conjugated to a protein carrier, as is the case for the conjugate Haemophilus influenzae type b (Hib) vaccines." (Nelson) Polysaccharides are large carbohydrates. A number of the antigens that we react to are polysaccharides, including those for Hib, PCV, and salmonella. The only two childhood vaccines that use polysaccharides are Hib and PCV. Both use conjugated protein carriers to allow the immune system to respond to the antigens before two years of age.
You can (and should) draw your own conclusions. As far as I can tell, the only thing that the proponents of delayed vaccination are referring to when they say the immune system isn't fully developed for two years is the inability of the infant below that age to produce antibodies to raw polysaccharide antigens. These antigens are not used in childhood immunizations, and infants -- even premature infants -- are born with the ability to develop an immune response.
"Current studies do not support the hypothesis that multiple vaccines overwhelm, weaken, or "use up" the immune system. On the contrary, young infants have an enormous capacity to respond to multiple vaccines, as well as to the many other challenges present in the environment. By providing protection against a number of bacterial and viral pathogens, vaccines prevent the "weakening" of the immune system and consequent secondary bacterial infections occasionally caused by natural infection. "