White Blood Cell and Platelet Disorders

Infectious Mononucleosis

Not all WBC abnormalities are cancerous. Infectious mononucleosis is a contagious viral infection caused by the Epstein-Barr virus (EBV). Clinically, this is often identified by the presence of atypical lymphocytes and enlarged lymphocytes in a blood smear.

Exam Tip: If a clinical scenario mentions a patient with extreme fatigue, enlarged lymph nodes, and a blood smear showing "atypical lymphocytes," think of Epstein-Barr virus and Infectious Mononucleosis.

Platelet Disorders

Platelets are not whole cells but rather cytoplasmic fragments derived from megakaryocytes in the red bone marrow. Their production is regulated by the hormone thrombopoietin. Their primary role is hemostasis—forming temporary platelet plugs to stop bleeding.

Thrombocytopenia and Thrombocytosis

• Thrombocytopenia: This is a deficiency in the platelet count. It can result from decreased production (such as in aplastic anemia or B12 deficiency) or increased destruction. Clinical signs include easy bruising and petechiae—tiny red or purple dots on the skin caused by minor hemorrhages.

• Thrombocythemia (Thrombocytosis): This is an excessive platelet count, often triggered by infection, inflammation, or iron deficiency.

Platelet Dysfunction

In some cases, the platelet count may appear normal, but the cells are functionally defective. Platelet dysfunction occurs when platelets are present in sufficient numbers but fail to adhere to vessel walls, activate, or aggregate correctly to form a plug.

Blood-Clotting Disorders

Coagulation is a complex cascade involving both cellular components and plasma proteins (clotting factors). Disorders are generally categorized by whether the blood fails to clot or clots too much.

Bleeding Disorders

Bleeding disorders result in insufficient clotting, leading to prolonged hemorrhage. A primary example is Hemophilia, a group of genetic disorders that are often X-linked:

• Hemophilia A: The most common form, caused by a deficiency in Factor VIII (approximately $80%$ of cases).
• Hemophilia B: Caused by a deficiency in Factor IX (approximately $20%$ of cases).
• Hemophilia C: A rarer and typically milder form caused by a deficiency in Factor XI.

Thrombophilia

On the opposite end of the spectrum is thrombophilia, a condition characterized by excessive clotting. This increases the risk of dangerous thrombosis (clot formation within a vessel) and embolism (a clot that travels through the bloodstream to lodge in another vessel). A well-known example of a thrombophilic condition is Factor V Leiden.

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