Chapter 11: Immunity

Immunity is the protection against disease provided by the body’s internal defense or immune system. Immunity can be defined as a complex biological system endowed with the capacity to recognize and tolerate whatever belongs to the self, and to recognize and reject what is foreign (non-self). 

Cells of Immunity

The cells of immune system originate from the bone marrow. There are two groups of cells involved and they are:

  • Phagocytes
  • Lymphocytes

Phagocytes

Phagocyte, type of cell that has the ability to ingest, and sometimes digest, foreign particles, such as bacteria, carbon, dust, or dye. It engulfs foreign bodies by extending its cytoplasm into pseudopods (cytoplasmic extensions like feet), surrounding the foreign particle and forming a vacuole

Phagocytes are white blood cells that are produced continuously in the bone marrow. They are stored in the bone marrow before being distributed around the body in the blood. There are two main types of phagocyte, each with a specific mode of action. The two types are:

    • Neutrophils
    • Macrophages

Neutrophils are a type of white blood cell. They make up the biggest number of all kinds of white blood cells. They kill and digest bacteria and fungi to help your body fight infections and heal wounds. They are short lived cells.

Macrophages are specialized cells involved in the detection, phagocytosis and destruction of bacteria and other harmful organisms. In addition, they can also present antigens to T cells and initiate inflammation by releasing molecules (known as cytokines) that activate other cells.  macrophages take different names according to their tissue location, such as osteoclasts (bone), alveolar macrophages (lung), microglial cells (brain), histiocytes (connective tissue), Kupffer cells (liver), Langerhans cells (LC) (skin), etc.

Both carry out Phagocytosis; process by which certain living cells called phagocytes ingest or engulf other cells or particles

Lymphocytes

Lymphocytes are a type of white blood cell. They play an important role in your immune system, helping your body fight off infection. Many underlying medical conditions can cause lymphocytosis. High lymphocyte blood levels indicate your body is dealing with an infection or other inflammatory condition. There are two types of lymphocytes (with different modes of action). The two types of lymphocytes are:

    • B-lymphocytes (B cells)
    • T-lymphocytes (T cells)

B-lymphocytes (B cells)

B lymphocytes are part of the immune system and develop from stem cells in the bone marrow. Also called B cell.  By producing antibodies, B cells are main players in the protective immune response against pathogenic infections. In response to antigens, they mature into antibody-producing plasma cells or into memory B cells, which can quickly be reactivated following secondary challenge.

The main functions of B cells are:

  • to make antibodies against antigens,
  • to perform the role of antigen-presenting cells (APCs),
  • To develop into memory B cells after activation by antigen interaction.

T-lymphocytes (T cells)

T lymphocytes are part of the immune system and develop from stem cells in the bone marrow. They help protect the body from infection and may help fight cancer. Also called T cell and thymocyte. T cells are a part of the immune system that focuses on specific foreign particles. Rather than generically attack any antigens. T cells mature in thymus gland. T cells circulate until they encounter their specific antigen. As such, T cells play a critical part in immunity to foreign substances. There are 3 main types of T cells: cytotoxic, helper, and regulatory. T-lymphocytes differentiate into two main types of T cell:

  • helper T cells
  • killer T cells

Antibodies

  • Antibodies are globular glycoproteins called immunoglobulin
  • Antibodies have a quaternary structure (which is represented as Y-shaped), with two ‘heavy’ (long) polypeptide chains bonded by disulfide bonds to two ‘light’ (short) polypeptide chains
  •  Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses.
  • The antibody recognizes a unique molecule of the pathogen, called an antigen.
  • There are 5 types of heavy chain constant regions in antibodies (immunoglobulin) and according to these types; they are classified into IgG, IgM, IgA, IgD, and IgE. They are distributed and function differently in the body.

Function

  • Antibodies are secreted into the blood and mucosa, where they bind to and inactivate foreign substances such as pathogens and toxins (neutralization).
  • Antibodies activate the complement system to destroy bacterial cells by lysis (punching holes in the cell wall).
  • Antibodies facilitate phagocytosis of foreign substances by phagocytic cells.

Each antibody recognizes one specific antigen.
For example, an antibody that recognizes the mumps virus cannot recognize the measles virus. Conversely, an antibody that recognizes the measles virus cannot recognize the mumps virus. This feature is called “antibody specificity.”

Monoclonal Antibodies

A monoclonal antibody (mAb or moAb) is an antibody made by cloning a unique white blood cell. They are made in a laboratory to fight a particular infection. They are produced by fusing a plasma cell with a cancer cell to produce hybridoma, which divides repeatedly to form many genetically identical cells that all produce the same antibody. Monoclonal antibodies have been approved to treat cancer, cardiovascular disease, inflammatory diseases, macular degeneration, etc.

Monoclonal Antibody Production by Hybridoma method:

This process starts by injecting a mouse (or other mammal) with an antigen that provokes an immune response. A type of white blood cell, the B cell, produces antibodies that bind to the injected antigen. These antibody producing B-cells are then harvested from the mouse and, in turn, fused with immortal B cell cancer cells, a myeloma, to produce a hybrid cell line called a hybridoma,

(1) Immunization of a mouse
(2) Isolation of B cells from the spleen
(3) Cultivation of myeloma cells
(4) Fusion of myeloma and B cells
(5) Separation of cell lines
(6) Screening of suitable cell lines
(7) in vitro (a) or in vivo (b) multiplication
(8) Harvesting