Central University of South Bihar Panchanpur, Gaya, India

E-Learning Resources

Department of Biotechnology

NB: These materials are taken/borrowed/modified/compiled from various resources like research articles and freely available internet websites, and are meant to be used solely for the teaching purpose in a public university, and for serving the needs of specified educational programmes.

Dr. Jawaid Ahsan

Assistant Professor Department of Biotechnology

Central University of South Bihar (CUSB)

Course Code: MSBTN2004C04 Course Name: Biology of Immune System

The Blood System

Blood is slightly alkaline (pH = 7.40 ± .05) and slightly heavier than water (density = 1.057 ± .009). All blood cells are manufactured by stem cells, which live mainly in the bone marrow, via a process called hematopoiesis.

The stem cells produce hemocytoblasts that differentiate into the precursors for all the different types of blood cells. Hemocytoblasts mature into three types of blood cells: erythrocytes (red blood cells or RBCs), leukocytes (white blood cells or WBCs), and thrombocytes (platelets).

All of the major cells in the blood system are illustrated here.

Neutrophils (mostly), eosinophils, and basophils are polymorphonuclear leukocytes (PMNs or PMLs).

Normal Adult Blood Cell Counts

Red Blood Cells 5.0*106/mm3

Platelets 2.5*105/mm3

Leukocytes 7.3*103/mm3

Neutrophil 50-70%

Lymphocyte 20-40%

Monocyte 1-6%

Eosinophil 1-3%

Basophil <1%

About 2,400,000 RBCs are produced each second and each lives for about 120 days . They migrate to the spleen to die. Once there, that organ scavenges usable proteins from their carcasses. As a part of the mononuclear phagocyte system, spleen metabolizes hemoglobin removed from senescent erythrocytes. The globin portion of hemoglobin is degraded to its constitutive amino acids, and the heme portion is metabolized to bilirubin, which is subsequently shuttled to the liver for removal. A healthy male has about 5 million RBCs per mm3, whereas females have a bit fewer than 5 million.

Eosinophils deal with parasitic infections. Basophils are responsible for allergic response (releasing histamine). Neutrophils are mainly in charge of defending us from bacteria and fungus. Lymphocytes are common in the lymphatic system. There are three types: B cells, T cells and natural killer cells. Monocytes leave the blood to become macrophages (they allow a process called phagocytosis) and dendritic cells. If the number of leukocytes is higher than normal, the condition is called leukemia (cancer of the blood or bone marrow). If the number of leukocytes is lower, the condition is called leukopenia.

Haematopoiesis Haematopoiesis (from Ancient Greek: αἷμα, "blood"; ποιεῖν "to make") (or hematopoiesis in American English; sometimes also haemopoiesis or hemopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells (HSCs). In a healthy adult person, approximately 1011–1012 new blood cells are produced daily in order to maintain steady state levels in the peripheral circulation.

In developing embryos, blood formation occurs in aggregates of blood cells in the yolk sac, called blood islands. As development progresses, blood formation occurs in the spleen, liver and lymph nodes. When bone marrow develops, it eventually assumes the task of forming most of the blood cells for the entire organism. However, maturation, activation, and some proliferation of lymphoid cells occurs in secondary lymphoid organs. In children, haematopoiesis occurs in the marrow of the long bones such as the femur and tibia. In adults, it occurs mainly in the pelvis, cranium, vertebrae, and sternum.

Extramedullary In some cases, the liver, thymus, and spleen may resume their haematopoietic function, if necessary. This is called extramedullary haematopoiesis. It may cause these organs to increase in size substantially. During fetal development, since bones and thus the bone marrow develop later, the liver functions as the main haematopoetic organ. Therefore, the liver is enlarged during development.

Sites of haematopoiesis (human) in pre- and postnatal period.

The Lymph System The word ‘lymph’ is derived from the name of the Roman deity of fresh water, Lympha. Lymph is an alkaline (pH > 7.0) fluid that is usually clear, transparent, and colorless. It flows in the lymphatic vessels and bathes tissues and organs in its protective covering. Like blood, it is slightly heavier than water (density = 1.019 ± .003). The lymphatic system is part of the circulatory system, comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph directionally towards the heart. Lymph is a clear to yellowish watery fluid that is found throughout the body. It circulates through body tissues picking up fats, bacteria, and other unwanted materials, and filtering them out through the lymphatic system.

This fluid contains white blood cells, known as lymphocytes, along with a small concentration of red blood cells (or no RBCs) and proteins. It circulates freely through the body, bathing cells in needed nutrients and oxygen while it collects harmful materials for disposal. Likewise, the lymph formed in the digestive system called chyle is rich in triglycerides (fat), and looks milky white.

Unlike the cardiovascular system, the lymphatic system is not closed and has no central pump. Lymph transport, therefore, is slow and sporadic. Despite low pressure, lymph movement occurs due to peristalsis.