Immune System 101

White blood cells, or leukocytes, are cells of the immune system defending the body against both infectious disease and foreign materials. They are found throughout the body, including the blood and lymphatic system, making up approximately 1% of blood in a healthy adult.

Granulocytes are composed of the following cells:

Neutrophils/polymorphonuclear cells (PMNs): are “the scavenger cells” that make up 50 – 70% of the White Blood Cells. The Neutrophils/PMNs are non-specific, phagocytic in property that digest and eliminate intruding microorganisms, debris, injured/dead host cells. They are usually attracted to sites of tissue injury/inflammatory reaction by chemotactic factors.

Eosinophils: make up approximately 1 – 3% of the White Blood Cells. The Eosinophils are also phagocytic in property and are usually increased in number during parasitic infections. They can contribute to inflammatory, allergic reactions.

Basophils: are rare among white blood cells, which make up less than 1% of the White Blood Cells. Unlike Neutrophils and Eosinophils, Basophils are non-phagocytic, contain histamine and other mediators in their basic granules. They usually facilitate allergic, anaphylaxis reactions.

Monocytes: are mononuclear, non-specific phagocytic cells that make up 5-10% of the circulating White Blood Cells. There are three functions with regards to Monocytes:

1. Non-specific phagocytic in property that digest and eliminate intruding microorganisms, debris, injured/dead host cells.

2. Antigen-Presenting Cell (APC), which digested antigens associate intracellularly with major histocompatibility (MHC) complex molecules, carried to and display on cell surface. In other words, upon finishing digesting intruding foreign cells, monocytes “display” pieces of antigen on the surface of cells and show to other phagocytes to promote Adaptive (antigen-specific) immune responses*

3. Secretory cell: participate in both non-specific/inflammatory and specific response upon phagocytosis. In other words, Monocytes secrete enzymes, release chemical signals (chemokines) to summon neutrophils, and finally , release cytokines which is the communication between cells.

Monocytes are generally known as the tissue Macrophages.

Lymphocytes: makes up 20 – 40% of circulating White Blood Cells, responsible for antigen specific immune response and innate defense in viral and other cell-associated infections. There are three categories of cells in lymphocytes:

1. Natural Killer Cells (NK Cells): (1 – 4% circulating white blood cells, 10-15% of lymphocytes)

The ability of the NK cells is to kill wide range of tumor cells in absence of previous exposure to tumor. They have no antigen specificity and have no “memory” on intruding pathogens. NK cells are equivalent of granulocytes which both serves as the frontline of any foreign infections.

2. B and T Lymphocytes: unlike the NK cells, they are responsible for antigen specific immune responses.

(a) B cells: (10-15% of the lymphocytes), produced in the bone marrow, migrated and matured in the spleen as B Lymphocytes. Their principal function is to circulate in the blood and lymph performing the role of immune surveillance by making antigens against soluble antigens.

(b) T Cells: (75 – 80% of lymphocytes) mature in thymus, flat, bilobed organ above the heart. There are two types of T cells

(b1) T Helper CD4+: are the “middleman” of an adaptive immune system. Once activated, they divide rapidly and secrete small “messages between cells” proteins called cytokines that regulate or “help” the immune response.

(b2) Cytotoxic T killer, CD 8+ cells: destroy virally infected cells and tumor cells.

In addition, the cell surface molecules play critical roles in immune response.

a. Immune system has ability to distinguish self vs non-self depends on MHC (Major Histocompatibility Complex)

b. Immune system has the ability to recognize molecules which are foreign (depending on antigens) Lymphocytes are the only cells with the ability to specifically recognize and respond to antigen; therefore, have cell surface molecules which serve as antigen receptors.