TYPES OF IMMUNITY

Terms-

Basic terms used in immunology-

1. Immunology- Immunology is a branch of biology that covers the study of the immune system. 
2. Immunity- Immunity is the ability of an organism to resist infection by a pathogen.
3. Immune system- The array of cells, tissue, and organs that carry out this activity is called the immune system.

Immunity is divided into two categories:

1. Innate immunity-
2. Adaptive immunity-

1. Innate immunity-

Innate (native/natural) immunity is present since birth and consists of many factors such as - 

• It is also called natural immunity.
• It operates any foreign molecules and pathogens.
• It provided the first line of defense against pathogens.
• It is not specific to any one pathogen.
• It does not rely on previous exposure to a pathogen.
• Innate immune response has no memory.
• It consists of physical, chemical, and cellular defense against pathogens.
• The main purpose of the innate immune response is to immediately prevent the spread of pathogens.

Elements of innate immunity-

1. Physical barriers

• Physical barriers are the first line of defense against microorganisms. It includes skin and mucous membranes. Most organisms and foreign substances cannot penetrate intact skin but can enter the body if the skin is damaged.
• Secondly, the acidic pH of sweat and sebaceous secretions and the presence of various fatty acids and hydrolytic enzymes like lysozymes inhibit the growth of most microorganisms.
• Similarly, respiratory and gastrointestinal tracts are lined by mucous membranes. The mucous membrane entraps foreign microorganisms. The respiratory tract is also covered by silica, which are hair-like projections of the epithelial cell membranes. The synchronous movement of the cilia propels mucus-entrapped microorganisms out of these tracts.
• Tears contain lysozymes, lactoferrin, IgA, and thus provide chemical as well as physical protection.

2. Chemical mediator

• A variety of chemicals mediate protection against microbes during the period before adaptive immunity develops.
• The molecules of the innate immune response include complement proteins, cytokine, pattern recognition molecules, acute-phase proteins, cationic peptides, enzymes like lysozymes, and many others.

Compement proteins-

• The complement proteins are soluble proteins /glycoproteins that are mainly synthesized by the liver and circulate in the blood and extracellular fluid. They were originally identified by their ability to amplify and complement the action of antibodies hence the name complement. The complement system is composed of over 30 serum proteins. Activation of complement proteins in response to certain microorganisms results in a controlled enzymatic cascade, which targets the membrane of the pathogenic organisms and leads to their distraction.

Cytokines- 

• The term cytokine is a generic term for any low molecular weight soluble protein or glycoprotein released by one cell population which acts as an intercellular mediator. It includes monokines, lymphokines, interleukins, interferon, and others.
• Cytokines are required for the immunoregulation of both innate and adaptive responses.
• Interferons are Cytokines made by cells in response to various infections, which essentially induce a generalized antiviral state in surrounding cells.
• Chemokines are small, positively charged secreted proteins that have a central role in guiding the migrations of various types of white blood cells. They bind to the surface of endothelial cells, and to negatively charged proteoglycans of the extracellular matrix in organs. By binding to the G-protein-linked receptor on the surface of specific blood cells, Chemokines attract these cells from the bloodstream into an organ, guide them to specific locations within the organ, and then help stop migration.

Pattern recognition molecule-

• Many molecules involved in innate immunity have the ability to recognize Pathogen-Associated Molecular Pattern (PAMP) for the initial detection of microbes. 
• PAMS is a microbe-specific molecular signature. PAMS is recognized by pattern-recognition receptors (PRRs). 
• Mammals have several distinct classes of pattern recognition receptors (PRRs) including Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), Nod-like receptors (NLRs), AIM2-like receptors (ALRs), and C-type lectin receptor (CLRs). 
• Among these, TLRs were the first to be identified, and are the best characterized.

Acute-phase proteins- 

• Acute-phase proteins are a heterogeneous group of plasma proteins mainly produced in the liver as the result of a microbial stimulus. They include C-reactive protein (CRP), serum amyloid protein A (SAA), and mannose-binding protein (MBP). Cytokines released by macrophages upon activation by bacteria stimulate the liver to rapidly produce acute-phase microbes.

Cellular defenses-

• Many specialized cell types like neutrophils, monocytes, natural killer cells participate in innate host defense mechanisms. Once a pathogen invades the physical and chemical barriers, these specialized cells play an important role in protection. Phagocytosis is a fundamental protective mechanism carried out by these cell types, neutrophils, macrophages, monocytes, and dendritic cells.

Inflammatory barrier- 

• Inflammation is an important nonspecific defense reaction to cell injury. The signs of inflammation are pain, redness (erythema), swelling (edema), and heat. Each of these is the result of specific changes in local blood vessels. 

2. Adaptive immunity-

• It is the second line of defense against the non-self pathogens.
• It is also known as specific or acquired immunity.
• It displays four characteristics.

1. Antigenic specificity-It is the ability to discriminate among different antigens.
2. Immunological memory-It is the ability to recall the previous contact with the foreign molecule.
3. Diversity- It is the ability to respond to different antigens.
4. Self and non-self recognition- It is the ability to recognize and respond to a molecule that is foreign.

Acquired immunity may be active and passive -

• Immunity can be acquired actively or passively.
• Active immunity is when the person is exposed to a foreign substance and the immune system responds.
• Passive immunity is when the antibody is transferred from one host to another.

• Passive memory is usually short-term, lasting between a few days and several months.
• Active immunity is long-term 
• Active and passive immunity further divided into two-

1. Natural
2. Artificial

• Naturally Acquired Active Immunity – when a person is naturally exposed to antigens, becomes ill, then recovers.

• Naturally Acquired Passive Immunity – involves a natural transfer of antibodies from a mother to her infant. The antibodies cross the woman's placenta to the fetus. Antibodies can also be transferred through breast milk with the secretions of colostrum.

• Artificially Acquired Active Immunity – is done by vaccination (introducing dead or weakened antigen to the host's cell).

• Artificially Acquired Passive Immunity – This involves the introduction of antibodies rather than antigens to the human body. These antibodies are from an animal or person who is already immune to the disease.

    The immune response of adaptive immunity-

• There are two arms of acquired immunity, which have different sets of participants and different purposes but one common aim is to eliminate the antigen.
• Of these two arms of the acquired immune response, one is mediated mainly by B-cells and circulating antibodies, which is referred to as humoral immunity.

• The other is mediated by T-cells that do not synthesize antibodies but instead synthesize and release various Cytokines that affect other cells termed cellular or cell-mediated immunity. 

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