BASICS OF IMMUNITY

[COVID-19 SERIES #3]

INTRODUCTION:

Almost every living being on our planet has some sort of ability to resist damage to itself from infections and toxins. This protection from internal and external agents can be present as a consequence of genetic coding in the body. Therefore, depending upon the genetic makeup it is present in different forms. The absence of a particular genetic characteristic makes the organism susceptible to harm. Thus, we have mice who are “genetic-knockouts” and vulnerable to certain insults which would not affect other normal mice. There are other types of immunity which is acquired following exposure to the deleterious agents. Thus, we can develop resistance to certain allergic substances after a couple of exposures. Immunity can also be passed in a passive manner. Thus, mothers could transfer immunity to their fetus/babies or it can be injected by plasma therapy.

The term “immune” is derived from the Latin word “immunis” which means “free from” or “exempt”. The immune system is the biological defense mechanism. It comprises of different cells, organs and tissues that work together to combat infection, cellular damage and disease.

There are different methods by which immunity can be classified. It can be regarded as innate, adaptive and passive. It can also be classified pathologically into humoral and cellular immunity.

INNATE IMMUNITY:

Innate immunity is the first line of defense against non-self pathogens. It is the natural immunity with which an individual is born. A healthy individual is able to ward off antigens due to programmed processes present inherently in the body. It could be in the form of antibodies, skin cells which prevent organisms from invading the body and chemicals in the blood-stream. These are non-specific defense mechanisms that are activated immediately or within hours following exposure to antigens.

Molecules such as lipopolysaccharides, mannose and teichoic acid present on the surface of organisms form the “pathogen-associated molecular patterns” (PAMPs). These molecules interact with specific receptors known as “pattern recognition receptors” (PRRs) and activate the innate immune system.

ADAPTIVE IMMUNITY:

Adaptive or active immunity is acquired over time as individuals are exposed to organisms or are immunized. This type of immunity is only found in vertebrates. Adaptive immunity is an antigen-specific immune response. This variety is more complex than innate immunity. Once the body is exposed to an antigen, certain specific antibodies against the antigen are produced. The body also develops a “memory” against the antigen making the response more efficient. This immunity is mediated by B and T cells following exposure to a specific antigen. Apart from memory, such a process also involves specificity and self/non-self recognition. 

The adaptive immune response is meant to attack non-self antigens. However, in certain situations there is an error and it starts to attack itself. Such a mechanism is seen in autoimmune diseases e.g. systemic lupus erythematosus, rheumatoid arthritis etc.

PASSIVE IMMUNITY:

This is “borrowed” from another source and is effective for a short period only. Antibodies in a mother’s breast milk provide a temporary immunity to the child against diseases to which the mother was exposed.

HUMORAL IMMUNITY:

Adaptive immune mechanisms function through two mechanisms: (a) humoral immunity (b) cellular immunity. The main difference between humoral and cellular immunity is that antigen-specific antibodies are produced in humoral immunity, whereas antibodies are not produced in cell-mediated immunity. Instead, T cells destroy the infected cells by inducing apoptosis.

Therefore, humoral immunity is also known as antibody-mediated immunity. B-cells, with the assistance of helper T-cells differentiate into plasma B-cells which produce antibodies against a specific antigen. The humoral immune system deals with antigens from pathogens freely circulating or outside the infected cells. Antibodies from B-cells bind to the antigens and neutralize them or cause lysis (destruction or dissolution of cells by lysis) or phagocytosis.

Humoral immunity involves BCR receptors and recognizes unprocessed antigens. Once the antigen-antibody interaction occurs, the antibodies destroy the pathogen by three mechanisms. They bind to specific molecules on the surface of the pathogen, neutralizing the pathogen. This neutralization prevents the pathogens from entering the cells. It also neutralizes bacterial toxins. The pathogens which are bound to the antibodies undergo phagocytosis by macrophages and other cells. This is called “opsonization”. Binding of antibodies to pathogens activates the complement system. The complement system proteins then bind to the antibody- bound pathogens and recruit phagocytic cells.

CELLULAR IMMUNITY:

Cellular immunity occurs within the infected cells and is mediated by antigen-specific T-lymphocytes. The antigens (pathogen) are expressed on the cell surface or on an antigen-presenting cell. helper T-cells release cytokines that help activated T-cells bind to the infected cells’ MHC-antigen (Major histocompatibility complex) complex and differentiate the T-cell into a cytotoxic T-cell. The T-cells secrete cytokines and the infected cell then undergoes lysis.