The human immune system is composed of three lines of defense, each with different barriers, cells and chemicals that protect us from infection. The second line of immune defense is nonspecific, meaning that it does not target specific pathogens (microbes that cause disease), but instead provides general protection against all invaders.
The second line of immune defense comes into play when pathogens succeed in getting into the body by penetrating the skin or mucous membranes. The components of this second line of defense consist of cells, antimicrobial chemicals, and processes, but no physical barriers. Many components of second line defense are contained, or originate, in the blood, including most of the white blood cells, also known as leukocytes.
* Blood and the Immune System *
Blood is composed of cells and portions of cells within a fluid medium called plasma. Plasma is mostly water, but also contains electrolytes, dissolved gases, nutrients, and proteins.
The cells and cell fragments in plasma are called “formed elements”, and include red blood cells and platelets.
There are three types of formed elements:
1. Erythrocytes – red blood cell, which carry oxygen & carbon dioxide in the blood
2. Platelets – which are involved in blood clotting
3. Leukocytes – white blood cells involved in defending the body against invaders
* Types of Leukocytes *
Leukocytes can be divided into two main groups:
~ Granulocytes ~
These white blood cells are characterized by the presence of granules (small particles that are easy to stain and see under a microscope) in their cytoplasm.
There are three types of Granulocytes:
1a. Neutrophils: These are the most abundant type of white blood cell, and the predominant cells in pus, accounting for its whitish appearance. Neutrophils react within an hour of tissue injury and are the hallmark of acute inflammation.
1b. Basophils: These are the least common granulocyte. When activated, basophils release histamine, proteoglycans (e.g. heparin and chondroitin), and proteolytic enzymes, all of which contribute to inflammation.
1c. Eosinophils: These cells primarily play a role in fighting viral infections, and are also considered to be the the main effector cells in allergic responses and asthma. Eosinophils also fight helminth (worm) colonization.
Neutrophils and Eosinophils can phagocytize (engulf and digest) pathogens whereas Basophils release chemical that help us fight infection.
~ Agranulocytes ~
These white blood cells are distinguished by the absence of granules in their cytoplasm.
There are two types Agranulocytes:
2a. Lymphocytes: Primarily involved in the third line of immune defense; specific immunity.
2b. Monocytes and Macrophages: Macrophages are phagocytic cells of the second line of defense. They ingest and destroy foreign matter such as microorganisms or debris via a process known as phagocytosis.
Macrophages originate in the bone marrow as Monocytes, and are called Macrophages once they migrate to the organs. Macrophages come in two varieties. Wandering macrophages leave the blood and phagocytize throughout the body, while fixed macrophages do not move throughout the body and often phagocytize within a specific organ.
* Sources *
Bauman, R. (2005) Microbiology. Pearson Banjamin Cummings.
Park Talaro, K. (2008) Foundations in Microbiology. McGraw-Hill.