2025 – PAGE 108 – ALLERGY & IMMUNOLOGY

PEDIATRIC LYMPHOcyte COUNTS

Pediatric lymphocyte counts are much higher than those of adults. The absolute (total) lymphocyte count in children under two years old should be over 3000/mm³. The lymphocyte percentage on the differential should be > 40%. If the value looks like a normal adult LYMPHOcyte count value, it may be too LOW and the patient may be LYMPHOPENIC so you should confirm by calculating the absolute lymphocyte count (ALC).

PEARL: LYMPHOcytes do NOT mean total WBC (leukocyte) counts!

T-CELL DEFICIENCIES AND COMBINED T-CELL/B-CELL DEFICIENCIES

PEARLS:

• THIS SECTION IS HEAVILY REPRESENTED ON THE EXAM!
• Make a strong effort to memorize the probable age of presentation for the various immunodeficiencies. That can be a HUGE clue if offered up by the ABP to help you narrow the answer down from two choices to one.
• T-CELL MEDIATED IMMUNITY includes activation of antigen-specific cytotoxic T-lymphocytes, macro­phages, and natural killer (NK) cells. The patient can get recurrent viral, fungal, and bacterial infections. For T-CELL or COMBINED T-CELL/B-CELL deficiencies, look especially for chronic or recurrent Candida infec­tions (mouth, nail, scalp).
• All patients with a T-cell defect are at an INCREASED RISK OF LYMPHOMA/CANCER as they age (curious DiGeorge, Hyper-IgM, SCID, wiXoTt Ostrich).
• SKIN TESTING with CANDIDA, MUMPS, TETANUS, and PURIFIED PROTEIN DERIVATIVE (AKA PPD, AKA TUBERCULIN SKIN TEST) can all be used to diagnose Delayed-type hypersensitivity ANERGY. This checks for Type IV, CELL-MEDIATED IMMUNITY associated with T-cell defects. This includes CD4 cells and problems that arise from HIV/AIDS.
• PEARL IN A PEARL: If you are presented with an AIDS or immunocompromised patient who had a negative PPD, consider further Quantiferon Gold.
• PEARL IN A PEARL: SKIN testing for tetanus is completely different from getting titers for tetanus. Titers are checking for antibody production (B-cell function).

SEVERE COMBINED IMMUNODEFICIENCY (SCID)

Severe combined immunodeficiency (SCID) is a T AND B CELL DEFICIENCY (hence the word COMBINED). With T and B cells reduced, the hallmark is a low lymphocyte count (LYMPHopenia). Look for an Absolute Lymphocyte Count (ALC) that is well below normal, remembering that normal lymphocyte counts below 2 years old are much higher (over 3000/mm³) than in older children and adults. These patients do NOT have LYMPHadenopathy, but they DO have a small thymus. The presence of a thymus means a bone marrow transplant is a viable treatment option. These patients get all kinds of infections (viral, bacterial, fungal, and opportunistic). The patient usually presents in the first 3–6 months with otitis media (OM), thrush, diarrhea, and dermatitis. There is complete absence of T-cell function (on fluorometric analysis of T, B, and NK cells). This condition usually presents closer to 3 months. In contrast, a pure B-cell deficiency presents around 6 months of age.

• Possible presentations: PCP, a viral pneumonitis that doesn’t resolve or recurrent candidiasis that seems to be refractory to treatment. For PCP patients, always consider HIV in the differential, too.
• LIVE VACCINES: Do not give SCID patients live vaccines!
• CURE: Bone Marrow Transplant (BMT)