increases the risk of excessive nerve cell death, from which there can be adaptation but not full recovery. New tracts may develop but new cells are unlikely to appear--or at best to a limited extent.
Aside from direct toxic effects, indirect brain inflammation and vascular damage by metabolic products of vaccine stress can also occur. Homocysteine is one of these. Many toxins can provoke release of free amino acids, including methionine, and thus induce increased homocysteine, a well known excitotoxin. Homocysteine can cause apoptosis and if high levels should occur during a time of accelerated development this might indeed cause harm. If the nerve activity is further accelerated by pertussis (or fluoride, which also stimulates G proteins) then the damage would be that much greater.
Auto-immune effects, can interfere with myelination and cause prolonged inflammation that magnifies damage. Thus an injury can become chronic and prevent recovery from otherwise minor episodes of brain inflammation and developmental dysregulation. This may be more common now that newborns are vaccinated for hepatitis B on their very first day of life!
This is all by way of conjecture about how pertussis vaccine in particular can alter neural development. Aside from direct toxic effects, indirect brain inflammation and vascular damage by metabolic products, such as homocysteine, can also occur. Any stress or toxin can provoke release of free amino acids, including methionine, and thus induce increased homocysteine. It may be as simple as that; however the necessary research in this area has yet to be done. The damage is known to occur; only the explanations lag behind.
Auto-immune inflammation can become chronic and prevent recovery from otherwise minor episodes of brain inflammation and developmental dysregulation. This may be more common now that newborns are vaccinated for hepatitis B on their very first day of life! Another possibility is that the brain remains a fetal organ for a considerable time after birth: it is not nearly complete in its development until at least three years. There are cycles of development of various parts of the brain: cerebellum in the first months; sensory organs soon after; and cerebral cortex last. The auditory cortex, site of language development, reaches a critical period between 12 and 30 months, just about the time scheduled for MMR vaccination and booster shots.
The MMR vaccinations in the second year are targeted against viral illness, specifically measles, mumps and rubella (MMR). The vaccines contain live but weakened virus and though it is less virulent than the 'wild' type viral infection, the vaccine virus can overload the immune and anti-inflammatory systems in vulnerable children, especially if they are depleted in nutrients, such as selenium, vitamin E, or glutathone. High levels of immune hormones can be induced by infection, and some of these, such as interferon-alpha and interluekin-6 and 10, can act as neurotoxins. Since neural systems development relies heavily on a process called apoptosis, pruning away neurons that are extraneous or in some way do not fit into the competitive process involved as axon growth cones seek their receptors, it is possible that during critical periods of accelerated development, a disruption of apoptosis could lead to excessive cell death and loss of neurons that would better have been preserved by a more efficient process. In other words, accidents can and certainly do occur.