IMMUNOLOGY-AN OVERVIEW

 

IMMUNOLOGY-AN OVERVIEW

1 MARK QUESTIONS

1.      Immunity

The power of a versatile defence system that has evolved to protect animals from invading pathogenic microorganisms and cancer is called immunity. It is able to generate an enormous variety of cells and molecules capable of specifically recognizing and eliminating an apparently limitless variety of foreign invaders like pathogens or antigens

2.      Innate immunity

Innate immunity is the less specific component that provides the rapid, first line of defence against infection. Most components of innate immunity are present before the onset of infection. Phagocytic cells, such as macrophages and neutrophils, barriers such as skin, and a variety of antimicrobial compounds synthesized by the host all play important roles in innate immunity by recognizing and clearing frequently encountered pathogens.

3.      Acquired/adaptive immunity

Adaptive immunity is the quicker, stronger and specific immunity against an antigen within five or six days after the initial exposure to that antigen. Exposure to the same antigen sometimes in the future results in a memory response. It is more effective in neutralizing and clearing the pathogen by utilizing antibodies or cell mediated mechanisms.The major agents of adaptive immunity are lymphocytes and the antibodies and other molecules they produce.

4.      Components of innate immunity

Phagocytic cells (macrophages, neutrophils, monocytes), NK cells, barriers (skin), Microbial probionts, Surface secretions (acids, sweat, sebum, mucus, proteolytic enzymes, lactoperoxidase in milk), Mechanical action (Peristalsis, Washing effect of tears, saliva and urine) etc.

5.      Phagocytes

Phagocytes are some specialized cells, such as blood monocytes, neutrophils, and tissue macrophages which uptake materials from its environment. In phagocytosis, a cell’s plasma membrane expands around the particulate material, (an antigen or whole pathogenic microorganisms) to form large vesicles called phagosomes which are later lysed by proteolytic secretions of the lysosomes in the cell.

 6.      NK cells

These are large granular lymphocytes and one of the null cells thatlack surface antigens
(T-cell receptors or immunoglobulin incorporated in their plasma membranes). These cells display cytotoxic activity against a wide range of tumour cells in the absence of any previous immunization with the tumour. They distinguish infected cells and tumours from normal and uninfected cells and induce infected cells to undergo apoptosis (cell death) by releasing cytotoxic granules.

7.      Inflammation

Tissue damage caused by a wound or by an invading pathogenic microorganism induces a complex sequence of events collectively known as the inflammatory response for the clearance of the invader. The sequence of events include vasodilation and increased blood supply to the site of infection, increase in capillary permeability and an influx of lymphocytes and serum proteins from the engorged capillaries into the tissue, and the phagocytosis of antigens. Four cardinal signs of inflammation are rubor (redness), tumor (swelling), calor (heat), and dolor (pain).

8.      Pus

During inflammation, as phagocytic cells (neutrophils and monocytes) accumulate at the site of infection and begin to phagocytose antigens or pathogens, phagocytes release lytic enzymes which can damage nearby healthy cells. The accumulation of dead cells, digested material, and fluid forms a substance called pus.

9.      Active immunity

Active immunity is the form of immunity that is induced in an individual by exposure to a foreign antigen either naturally or artificially. It is so called because the immunized individual plays an active role in responding to the antigen by eliciting his acquired immunity (proliferation of antigen-reactive T and B cells resulting in the formation of memory cells).

e.g., Normal immune response to an infection (natural) and immunity attained by vaccination (artificial)

10.  Passive immunity

The immunity against a particular antigen/pathogen conferred on an individual by transferring serum or lymphocytes from an immunized individual is called passive immunity. It is so called because the individual does not play active role in attaining immunity but he receives preformed antibody from another organism.

e.g., Immunity attained by maternal antibodies obtained through breast milk (natural) and immunity obtained by antivenom against snake bite (artificial).

11.  Vaccines

Vaccines are either killed or live attenuated pathogens or toxoids (inactive toxins). These are administered to individual either orally or through injections. These vaccines cannot actively infect cells or produce any harmful products but are soon neutralized by the acquired immunity of the individual because vaccines are killed or weakened pathogens or inactive toxins. During this immune response the individual develops memory cells which will later identify the same pathogen up on further exposure and neutralize it in a very short period of time.e.g., vaccines against typhoid, cholera, pertussis, plague, rabies, polio etc.

12.  Humoral immunity

Immunity provided by specific chemical substances called antibodies that are synthesised by plasma cells derived from B lymphocytes is called humoralimmunity. It is so called because the antibodies secreted circulate in the body fluids (humor) such as blood plasma and lymph.

13.  Cell-mediated immunity

Immunity provided by T cells and their secretions like cytokines and lymphotoxins is called cell-mediated immunity. Upon antigenic stimulation, effector T cells are generated. Both activated T_H cells and cytotoxic T lymphocytes (CTLs) serve as effector cells in cell-mediated immune reactions. T_H cells secretecytokines which can activate various phagocytic cells, enabling them to phagocytose and kill microorganisms more effectively. Cytokines also activate T_C cells to CTLs which kill altered self-cells (infected cells), virus infected cells and tumour cells.

 

   

2/4 MARK QUESTIONS

1.      Explain various levels of innate immunity

i. Species level-Immunity shown to a pathogen by all members of a species.
eg.,            1. Salmonella, Vibrio choleraeand Treponema pallidum are humanpathogens. But we are resistant to Rinderpest, which is pathogenic to cattles.

ii. Racial level-Immunity shared by most of the members of a genetically related population.
eg., 1. Algerian race sheep is resistant to anthrax. 2. Negroes in Central Africa resistant to falciparum malaria

iii. Individuallevel- Immunity shown to a pathogen by an individual within a given race and not shared by most other members of the race or species.

eg.,      1. Homozygous twins show similar degree of resistance to leprosy or tuberculosis

1.      Briefly explain inflammation

Tissue damage caused by a wound or by an invading pathogenic microorganism induces a complex sequence of events collectively known as the inflammatory response for the clearance of the invader. The sequence of events include vasodilation and increased blood supply to the site of infection, increase in capillary permeability and an influx of lymphocytes and serum proteins from the engorged capillaries into the tissue and phagocytosis of antigens.

Vasodilation and an increase in capillary permeability facilitates an influx of fluid and cells from the engorged capillaries into the tissue. An increase in the diameter of blood vessels and accumulation of exudate contribute to tissue redness (erythema) and tissue swelling (edema) and thereby an increase in tissue temperature. As phagocytes influx(chemotaxis)from the capillaries into the tissues at the site of infection and begin to phagocytose pathogens, they release lytic enzymes,which can damage nearby healthy cells.The accumulation of dead cells,digested material,and fluid forms a substance called pus.

2.      Describe briefly Four characteristic attributes of adaptive immunity

Adaptive immunity displays four characteristic attributes: 

1) Antigenic specificity, 2) Diversity, 3) Immunologic memory and 4) Self/nonself recognition.

The antigenic specificityof the immune system permits it to distinguish subtle differences among antigens. Antibodies can distinguish between two protein molecules that differ in only a single amino acid. The immune system is capable of generating tremendous diversityin its recognition molecules, allowing it to recognize billions of unique structures on foreign antigens. Once the immune system has recognized and responded to an antigen, it exhibits immunologic memory; that is, a second encounter with the same antigen induces a heightened state of immune reactivity. Because of this attribute,the immune system can confer life-long immunity to many infectious agents after an initial encounter.Finally,the immune system normally responds only to foreign antigens, indicating that it is capable of self/non-self recognition. The ability of the immune system to distinguish self from non-self and respond only to non-self molecules is essential, for the outcome of an inappropriate response to self-molecules can be fatal.

3.      Distinguish between innate immunity and adaptive immunity.

Innate immunity is the less specific component that provides the rapid, first line of defence against infection by the components which are present in the body before the onset of infection. Phagocytic cells, such as macrophages and neutrophils, barriers such as skin, and a variety of antimicrobial compounds synthesized by the host all play important roles in innate immunity by recognizing and clearing frequently encountered pathogens.

Adaptive immunity is the quicker, stronger and specific immunity against an antigen within five or six days after the initial exposure to that antigen.Exposure to an antigen will induce T cells to synthesize and secrete cytokines which in turn activates B cells to secrete antibodies (humoral immunity) and Tc cells to secrete lymphotoxins (cell mediated immunity). Antibodies bind neutralize antigens and lymphotoxins destroys the infected cells. Exposure to the same antigen sometimes in the future results in a memory response by retaining activated B cells (Memory cells) during prior infection. Memory cells can identify the antigen at once during later infections and bring about humoral immunity immediately after the infection. Because of this attribute,the immune system can confer life-long immunity to many infectious agents after an initial encounter.