Functions and Components of Blood

• Hemoglobin: This is the red pigment within RBCs made of proteins and heme. The heme portion contains iron, which is the specific component that binds to oxygen.
• Hematocrit: Also known as Packed Cell Volume (PCV), this measures the percentage of total blood volume composed of RBCs. Normal ranges are approximately 42–52% for men and 37–48% for women.

Leukocytes (White Blood Cells)

Leukocytes (WBCs) are the primary cells of the immune system. They are categorized into two main groups based on their appearance under a microscope: granulocytes and agranulocytes.

1. Granulocytes

These cells contain visible granules in their cytoplasm:

• Neutrophils: The most abundant WBC (60–70% of total WBC count). They are characterized by a segmented nucleus and pale lilac granules; their primary job is to phagocytize (engulf) bacteria.
• Eosinophils: Comprising 2–4% of WBCs, these cells have red cytoplasmic granules and a lobed nucleus. They function to destroy parasites and phagocytize allergens.
• Basophils: The rarest WBC (<1%), these have purple granules that often mask their nucleus. They release chemicals like histamine and heparin.

2. Agranulocytes

These cells lack visible granules:

• Lymphocytes: Making up 25–33% of WBCs, these cells are essential for immunity. They typically feature a large, round nucleus and scant cytoplasm.
• Monocytes: The largest of all WBCs (3–8%). When they leave the bloodstream and enter tissues, they become macrophages, which are highly effective at phagocytizing dead cells, pathogens, and debris.

Thrombocytes (Platelets)

Thrombocytes are not actually whole cells, but rather small fragments derived from large marrow cells called megakaryocytes. They lack a nucleus and are vital for hemostasis—the process of stopping bleeding.

Exam Tip: The Hemostasis Cascade Hemostasis occurs in three distinct steps:

1. Vascular Spasm: Smooth muscle in the broken vessel constricts to reduce blood loss.
2. Platelet Plug Formation: Platelets adhere to exposed collagen and to each other, creating a temporary seal.
3. Coagulation: A cascade of clotting factors results in the formation of a fibrin clot that secures the plug.

Blood Typing and Compatibility

Blood types are determined by the presence or absence of specific antigens on the surface of the erythrocytes.

The ABO Blood Group

The ABO system is based on the presence of A and B antigens. Compatibility is determined by whether a recipient has antibodies that would attack a donor's antigens.

The Rh Blood Group

The Rh (or D) antigen is another important marker.

• Rh+ individuals possess the D antigen and do not produce anti-D antibodies.
• Rh- individuals lack the D antigen. They do not produce anti-D antibodies naturally, but they will develop them if they are exposed to Rh+ blood (such as during a mismatched transfusion or certain pregnancy complications).

Universal Donor and Recipient

In clinical practice, we use the following classifications:

• Universal Recipient: Type AB+ is considered the universal recipient because it lacks antibodies against A, B, or D antigens.
• Universal Donor: Type O- is considered the universal donor because it lacks A, B, and D antigens, meaning it is unlikely to trigger an immune response in a recipient.

Note on Compatibility: These labels are not absolute. A "universal donor" could still cause a reaction if the donor's plasma contains antibodies (like anti-A or anti-B) that attack the recipient's RBCs.