1. Red Blood Cells (RBCs) or Erythrocytes

Function: Transport oxygen from the lungs to tissues and carry carbon dioxide back to the lungs for exhalation.

Structure: Biconcave discs without a nucleus, which maximizes their surface area for gas exchange.

Key Component: Hemoglobin, a protein that binds oxygen and gives blood its red color.

Lifespan: Approximately 120 days.

2. White Blood Cells (WBCs) or Leukocytes

Function: Protect the body against infections, foreign invaders, and abnormal cells.

Types:
Neutrophils: Fight bacterial and fungal infections.
Lymphocytes: Include T cells, B cells, and natural killer cells, important for immune responses.
Monocytes: Differentiate into macrophages and help in phagocytosis (engulfing pathogens).
Eosinophils: Combat parasitic infections and play a role in allergic responses.
Basophils: Release histamine and are involved in allergic and inflammatory responses.

Lifespan: Varies by type, from a few hours to several years.

3. Platelets or Thrombocytes

Function: Assist in blood clotting to prevent excessive bleeding.

Structure: Small, cell fragments derived from megakaryocytes.

Lifespan: 7–10 days.