Master the board step 3

Master the Boards: USMLE Step 3
Basic Science Correlate
Cromolyn and nedocromil work by stabilizing mast cells. They prevent
degranulation of mast cells so that histamine and leukotrienes are not
released. This mechanism is entirely preventive in nature: After exposure to
the allergen has stimulated the mast cells, cromolyn will not work.
Immunotherapy
For extrinsic allergens that cannot be avoided, desensitization therapy may
work. Use of beta blockers must be stopped before desensitization. If anaphylaxis
occurs during desensitization, then epinephrine is used—but if the
person is on a beta blocker, it will block the action of epinephrine. The need to
stop beta blockers before desensitization is a favorite Step 3 question.
Primary Immunodeficiency Syndromes
Common Variable Immunodeficiency (CVID)
CVID presents in both men and women and may only present when the patient
is an adult. Both CVID and X-linked agammaglobulinemia present with
recurrent episodes of sinopulmonary infections, such as bronchitis, sinusitis,
pneumonia, and pharyngitis. In addition, CVID causes a spruelike abdominal
disorder. There is malabsorption, steatorrhea, and diarrhea. Lymph nodes,
adenoids, and the spleen are present and may be enlarged.
Diagnostic Testing
The machinery to make immunoglobulins is intact. The nodes and both B and
T cells are present, but they do not make enough antibody. Hence total IgG
levels are low.
Treatment
CVID is treated with infusions of intravenous immunoglobulins.
X-Linked Agammaglobulinemia (Bruton’s)
This presents in male children with recurrent sinopulmonary infections.
Diagnostic Testing
Lymph nodes, adenoids, and the spleen are diminished in size or absent.
B cells are missing, as are the immunoglobulins.
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