Pregnant mothers pass antibodies (immunoglobulins) from their bloodstream, through the placenta and to the fetus. Immediately after birth, the newborn has high levels of the mother's antibodies. However, during the next couple of months, the infant's antibodies steadily decrease. When healthy babies are about two to three months old, the immune system will start producing its own antibodies. During this time, when babies are about three to four months of age, they will experience the body's natural low point of antibodies in the bloodstream. This is because the maternal antibodies have decreased, and young children, who are producing antibodies for the first time, produce the antibodies at a much slower rate than adults. Once healthy babies reach six months of age, their antibodies are produced at a normal rate.
Children who have THI continue to have low antibody levels after six months of age because the immune system is slow to produce sufficient levels of new antibodies. This is because the B-cells, which are responsible for antibody production, do not develop properly.
Healthy B-cells will mature into plasma cells and then produce antibodies against specific microbes. Individuals who have THI have normal B-cells numbers, but the B-cells do not mature appropriately into plasma cells. Therefore, the antibodies are not produced.
The reason for B-cell developmental abnormalities is unknown. There may be a hereditary component to THI, which causes problems with the conversion of B-cells to plasma cells. Some children with THI have family members who suffered from the same condition. However, many children have no family history of THI or other immune disorders.
Symptoms of THI vary. Some children may experience few or no symptoms while others may experience recurrent infections. In general, children who have THI suffer from milder infections that are caused by more typical bacteria and viruses than children with more severe immune deficiencies.
Common symptoms of THI include, recurrent ear infections or non-infectious inflammation of the middle ear, recurrent bronchitis, frequent sinusitis (infection or inflammation of the sinuses), bacterial infection (like pneumonia) and infections of the skin or meningitis (infection of the membranes that cover the spinal cord and brain).
General: THI is usually suspected if a child experiences recurrent infections past the age of six months. A blood test can indicate low levels of antibodies in the bloodstream. However, this is a nonspecific diagnostic test because many other immune disorders cause low levels of antibodies. A definitive diagnosis can only be made once the condition has resolved on its own.
Blood test: A blood test can measure the amount of antibodies present in the blood. If the antibody levels are less than what is considered normal for children of the same age, the child may have THI. However, low levels of antibodies are nonspecific because it can be the result of any other immunodeficiency disorder.
The blood sample can also be analyzed for antibodies that were produced in response to a vaccination. In general, most children with THI respond normally to vaccines (which is why they may have milder infections than children who have other immune disorders).
Retrospective diagnosis: A definitive diagnosis of THI is a retrospective diagnosis that can only be made after the child's antibody levels have increased to normal. Therefore, patients' antibody levels should be evaluated regularly to detect any change.