Symptoms & Emergency Warning Signs

Recognizing the symptoms of Transcobalamin II (TCII) deficiency is critical for early intervention. Because your baby’s cells cannot access Vitamin B12, the symptoms often look like a general “failure to thrive” or a serious infection ^[1][2].

Most infants with this condition begin to show signs between birth and 4 months of age, though some may not be diagnosed until up to 7 months ^[3][2].

Common Symptoms to Watch For

While every child is different, the lack of cellular B12 typically affects the blood, the digestive system, and the brain. You may notice:

• Pallor (Extreme Paleness): This is caused by megaloblastic anemia, where the body produces large, immature red blood cells that cannot carry oxygen effectively ^[1][2].
• Feeding and Growth Issues: This includes failure to thrive (poor weight gain), persistent vomiting, or diarrhea ^[4][1].
• Low Muscle Tone (Hypotonia): Your baby may feel “floppy” or have difficulty holding their head up ^[2][5].
• Lethargy: Extreme tiredness or a lack of energy that goes beyond normal infant sleepiness ^[2].
• Mucosal Ulcers: Small, painful sores in the mouth or on other mucous membranes ^[4].

Evaluating Symptoms at Home

It is natural to be on high alert after a diagnosis. While a mild, single episode of spitting up or a slightly “off” day without fever or breathing issues can be monitored, you should never delay emergency evaluation if your infant with immune dysfunction is beginning to deteriorate. If your child has poor feeding, repeated vomiting or diarrhea, fewer wet diapers, or unusual sleepiness, seek medical care immediately.

When to Seek Emergency Care

Certain symptoms in an infant with TCII deficiency are “red flags” that require immediate medical evaluation at an emergency department. Do not wait for a metabolic center to open; go to the nearest capable hospital.

1. Severe Respiratory Distress

If your baby is struggling to breathe, has a bluish or grayish tint to their skin or lips, or is breathing very rapidly, call emergency services immediately. TCII deficiency can weaken the immune system, making infants vulnerable to opportunistic infections—germs that don’t usually make healthy people sick ^[6]. Specifically:

• Cytomegalovirus (CMV) and Pneumocystis jirovecii pneumonia have been documented in infants with TCII deficiency ^[6].
• These infections can lead to Acute Respiratory Distress Syndrome (ARDS), a life-threatening lung condition where the lungs cannot provide enough oxygen to the body ^[6].

2. Worsening After Treatment Starts (IRIS-like Reaction)

In rare cases, a serious complication can occur shortly after starting high-dose B12 treatment. This has been reported as an Immune Reconstitution Inflammatory Syndrome (IRIS)-like reaction ^[6].

• The Cause: As the B12 supports immune recovery, the body may suddenly start a massive inflammatory response against existing (but previously “hidden”) infections ^[6].
• The Warning Sign: If your baby’s breathing or general condition suddenly worsens after they begin treatment, seek emergency care immediately. Sudden worsening after treatment is an emergency with many possible causes (including standard infections or anemia). Do not assume IRIS on your own; your medical team will evaluate and decide if specific treatments, like corticosteroids, are needed ^[6].

3. Neurologic and Immune Red Flags

• Seizures or Tremors: Any rhythmic shaking or unusual movements ^[7][2].
• Developmental Regression: Losing skills they already had, such as rolling over or making eye contact ^[7].
• Fever: Because your baby may have neutropenia (low white blood cell counts), their body cannot fight infections well ^[2]. Follow an age-specific fever instruction: for example, a rectal temperature of 38°C (100.4°F) or higher in an infant younger than 3 months requires immediate emergency evaluation. Defer to the child’s individualized immunology/metabolic plan for exact thresholds.
• Extreme Lethargy: If your baby is difficult to wake up or seems completely unresponsive.

Note: A “normal” blood test result for Vitamin B12 can be misleading. In TCII deficiency, the B12 is in the blood but can’t get into the cells. Therefore, a normal B12 level does not mean your baby is safe from these symptoms or emergencies ^[4][8].