A number of conditions can weaken the immune system of children. Some children are born with a weakened immune system, while other children's immune systems may have become weakened by either infections or medications. In any case, these children often require additional vaccines or a modified immunization schedule to compensate for their reduced capacity to protect themselves from germs.

Primary Immunodeficiency

Children may have a weak immune system for many reasons. If a child is born with a weak immune system, it is called primary immunodeficiency. This means that nothing has weakened the immune system after birth.

Most primary immunodeficiency occurs in children who have a problem making a particular type of chemical to fight germs. These children derive some protection from germs during the first six months of life because antibody (protective chemicals) from the mother is transferred through the placenta prior to birth to protect the baby. In addition, a breast-feeding mother confers even more protection for her infant because the breast milk contains rich amounts of protective protein that can ward off infections in babies.

Secondary Immunodeficiency

Secondary immunodeficiency means that the immune system has been weakened after birth. A slew of problems could weaken the immune system in children, including cancers (such as leukemia and lymphoma), chemotherapy for cancers, radiation treatment for cancers, AIDS, medications designed to suppress the immune system (necessary for children who have autoimmune diseases or children who had organ transplantation). Children who have chronic kidney or liver problems also have weakened immune systems because of problems making or retaining protein in the body.

Leukemia and lymphoma are some of the most common childhood cancers. These conditions can lower the immune system because these cancer cells either originate from inside the bones or infiltrate the bones when cancer spreads. In addition to supporting and protecting your body, your bones also serve as the headquarters for your body's defense system. Think of the bones as your body's Pentagon. When cancer cells multiply inside the bones, they squeeze out the normal defense cells in the bones. As these defense cells die out, the overall immune system takes a devastating blow.

Most chemotherapy and radiation regimens are designed to prevent fast-growing cells from multiplying, because cancer cells spread by rapidly making copies of themselves. Unfortunately, today's chemotherapy treatment does not distinguish between multiplying cancer cells and healthy growing cells. When these medications stop cells from multiplying, all the healthy cells that need to grow rapidly in the body are also affected. These include the hair cells, the cells lining the intestines, blood cells, and the immune system cells. Patients undergoing chemotherapy have an extremely weak or nonexisting immune system. They are completely vulnerable to every infection. These individuals need to live in a super-clean environment, and they cannot tolerate getting any vaccines containing live viruses.

Children with AIDS have a weakened immune system because HIV specifically infects immune cells. When immune cells are infected with HIV, they can no longer carry out their usual role of protecting the body. Vaccines containing live viruses can be detrimental for people with AIDS.

People who take corticosteroids because they have conditions that result from chronic inflammation also have weakened immune system. Many children are taking steroids because of severe asthma or allergy problems. Unfortunately, the steroid not only reduces the inflammation in the body, but it also suppresses the immune system.

Finally, people who have had organ transplantations need to permanently restrain their immune system by taking a powerful immunosuppressant. If their immune system is allowed to function unhindered, their body would recognize the transplanted organ as foreign and their immune system would unleash an attack on the foreign organ. This would lead to organ rejection, and death is almost inevitable unless the process can be stopped or they get a new organ. Being in a constant state of immunodeficiency, these patients typically cannot tolerate having live virus injected into their bodies.

Modification to the Immunization Schedule

As mentioned earlier, most vaccines containing live viruses should be avoided by children with defective immune systems. Even though the germs in these vaccines are significantly weakened and pose no threat to healthy children, these germs are nevertheless alive and they can cause life-threatening infection when the immune system is nonexistent or severely weakened. Vaccines that include live viruses are the MMR combination vaccine, the chickenpox vaccine, and the nasal form of the flu vaccine.

Another reason why the immunization schedule needs to be modified for these children is that, due to their underlying problem with their immune system, certain vaccines may not work at all because their immune system cannot be triggered by using traditional vaccines. Administering vaccines to these children would not do them any good, and it exposes them to unnecessary risks. Since the circumstances surrounding each individual is unique, consult your child's doctor for the best strategy to protect your child from infections.

Finally, many children with a weak immune system may require additional vaccines to protect them due to their deficiency. Children without a working spleen especially need protection against germs that can invade their bloodstreams, because the spleen is an important component of the body's defense against germs. Specifically, the Hib conjugate vaccine, the pneumococcal conjugate vaccine, and the meningococcal conjugate vaccine are very important for children without a working spleen. Please consult the earlier part of this chapter for a comprehensive reference if your child has a unique medical condition that may predispose her to certain infections.