Introduction to Immunity

 IMMUNITY

Overall ability of the host body to fight against the disease-causing organisms, conferred by the immune system is called immunity.

Immunity is of two types: (i) Innate immunity and (ii) Acquired immunity.

Innate Immunity

 Innate immunity consist of four types of barriers:

1. Physical barriers : Skin on our body is the main barrier which prevents entry of the micro-organisms. Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes entering our body
2. Physiological barriers : Acid in the stomach, saliva in the mouth, tears from eyes–all prevent microbial growth.
3. Cellular barriers : Certain types of leukocytes (WBC) of our body like polymorpho-nuclear leukocytes (PMNL-neutrophils) and monocytes and natural killer (type of lymphocytes) in the blood as well as macrophages in tissues can phagocytose and destroy microbes.
4. Cytokine barriers : Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection. 

Acquired Immunity

Acquired immunity, on the other hand is pathogen specific. It is characterised by memory. This means when our body encounters a pathogen for the first time it produces a response called primary response which is of low intensity while the Secondary response is of greater intensity. 

The primary and secondary immune responses are carried out with the help of two special types of lymphocytes present in our blood, i.e., B-lymphocytes and T-lymphocytes. 

The B-lymphocytes produce an army of proteins in response to pathogens into our blood to fight with them. These proteins are called antibodies. The T-cells themselves do not secrete antibodies but help B cells to produce them.

Each antibody molecule has four peptide chains, two small called light chains and two longer called heavy chains.  Hence, an antibody is represented as H2L2. Different types of antibodies are produced in our body. IgA, IgM, IgE, IgG are some of them. 

Humoral immunity is mediated by B-cells and involves the production of antibodies that fight pathogens in extracellular fluids.

Cell-mediated immunity is driven by T-cells and focuses on destroying infected host cells, cancerous cells, and intracellular pathogens.  

Active and Passive Immunity 

When a host is exposed to antigens, which may be in the form of living or dead microbes or other proteins, antibodies are produced in the host body. This type of immunity is called active immunity.

Active immunity is slow and takes time to give its full effective response

When ready-made antibodies are directly given to protect the body against foreign agents, it is called passive immunity.

What are Lymphocytes 

The two major populations of lymphocytes—B lymphocytes (B cells) and T lymphocytes (T cells)

B - LYMPHOCYTES

• B lymphocytes mature within the bone marrow; when they leave it, each expresses a unique antigen-binding receptor on its membrane.
• This antigen-binding or B-cell receptor is a membrane-bound antibody molecule.
• Anti bodies are glycoproteins that consist of two identical heavy polypeptide chains and two identical light polypeptide chains. Each heavy chain is joined with a light chain by disul fide bonds, and additional disulfide bonds hold the two pairs together.  
• The amino-terminal ends of the pairs of heavy and light chains form a cleft within which antigen binds.
• When a naive B cell (one that has not previously encountered antigen) first encounters the antigen that matches its membrane bound antibody, the binding of the antigen to the antibody causes the cell to divide rapidly; its progeny differentiate into memory B cells and effector B cells called plasma cells. 
• Memory B cells have a longer life span than naive cells, and they express the same membrane-bound antibody as their parent B cell.
• Plasma cells produce the antibody in a form that can be secreted and have little or no membrane-bound antibody.
• Although plasma cells live for only a few days, they secrete enormous amounts of antibody during this time. It has been estimated that a single plasma cell can secrete more than 2000 molecules of antibody per second. Secreted antibodies are the major effector molecules of humoral immunity.

T - LYMPHOCYTES

• T lymphocytes also arise in the bone marrow. Unlike B cells, which mature within the bone marrow,T cells migrate to the thymus gland to mature.
•  During its maturation within the thymus,the T cell comes to express a unique antigen-binding molecule,called the T-cell receptor, on its membrane.
• Unlike membrane-bound antibodies on B cells,which can recognize antigen alone, T-cell receptors can recognize only antigen that is bound to cell-membrane proteins called major histo compatibility complex (MHC) molecules. 
•  There are two major types of MHC molecules: Class I MHC molecules, which are expressed by nearly all nucleated cells of ver tebrate species, consist of a heavy chain linked to a small invariant protein called 2-microglobulin. 
• Class II MHC molecules, which consist of an alpha and a beta glycoprotein chain, are expressed only by antigen-presenting cells.
• When a naive T cell encounters antigen combined with a MHC molecule on a cell, the T cell proliferates and differentiates into memory T cells and various effector T cells.
• There are two well-defined subpopulations of T cells: T helper (TH) and T cytotoxic (TC) cells.
• Although a third type of T cell, called a T suppressor (TS) cell, has been postulated, recent evidence suggests that it may not be distinct from TH and TC subpopulations.
• T helper and T cytotoxic cells can be distinguished from one another by the presence of either CD4 or CD8 membrane glycoproteins on their surfaces.
• After a TH cell recognizes and interacts with an anti gen–MHC class II molecule complex, the cell is activated—it becomes an effector cell that secretes various growth factors known collectively as cytokines.
• The secreted cytokines play an important role in activating B cells, TC cells, macrophages, and various other cells that participate in the immune re sponse. 
•  Differences in the pattern of cytokines produced by activated TH cells result in different types of immune response.

Refrences : 

1. Kuby Immunology 6th Edition.
2. NCERT Std 12 biology textbook