Option 1 -> Erythrocytes are red blood cells that primarily transport oxygen throughout the body, not involved in fighting infections.
Option 2 -> Plasma is the liquid component of blood that carries nutrients, hormones, and waste products, but doesn't directly fight infections.
Option 3 -> Platelets are blood cells responsible for blood clotting and wound healing, not for fighting infections.
Option 4 -> Leukocytes are white blood cells that form the body's immune defense system and actively fight against pathogens and infections.
Hence, Option 4: Leukocytes -> White blood cells (leukocytes) are the primary component of the immune system in blood, producing antibodies and directly attacking foreign invaders like bacteria, viruses, and other pathogens -> correct