Immunity and its types – Innate and Acquired

All living organisms are connected in a complex web of relationship like symbiotic (mutual relationship), parasitic (utilize host material and cause damage), pathogenic (utilize host material and may kill it) and Commensalism (where one partner gets benefit while other is not affected). The human body provides ideal habitat for pathogenic organisms and hence, we get bacterial or viral fever and fungal infections. We, the vertebrates have evolved with several mechanisms that allow us to protect and fight with such pathogenic organisms and we call it as Immunity.

 Immunology is the branch of biological Science that deals with the study immune responses to the foreign substance and their role in the resisting infection. The fascinating facts about Immunity is that It can differentiate between self (own body cells or molecules) and non-self (foreign molecules); It also possesses immunological memory where it remembers the entire encountered foreign particle. And this is one of the working principles of vaccination.

Immunity is not at all one man Army. In fact, it is the combined efforts of various cells and organs. Based on the structure and function, the immune cells and organs are divided. The immune cells kill the pathogen. We will discuss more about it forthcoming articles. This article is dedicated to understand the concept and types of Immunity.

Immunity is defined as the ability of the body to recognize or destroy and eliminate harmful foreign substances, including micro-organisms. Immunity is classified into two types based on the mechanism of killing pathogens –  

• Innate  immunity/ non – special
• Acquired Immunity/ special Immunity

 Innate Immunity and its characteristics-

• Innate immunity is the inborn capacity of the body to resist the invasion of microorganisms into it.
• It is present from birth, hence called inborn immunity.
• It is also called natural immunity.
• It is relatively slow in action.
• It is non- specific   i.e. it operates in the same manner against all types of foreign particle.  
• Innate immunity operates through various types of barriers that prevent entry of foreign agent into the body.
• If the pathogens enter the body ,they are quickly killed by other components of innate immunity.

Barriers of Innate Immunity –

The primary protection from pathogenic organisms is provided by the physical barrier by preventing it entering the body. The physical barriers are also called as first line of defence.

A) Anatomical Barrier –

(1) Skin barriers :- 

1. It prevents the entry of microorganisms into the body.
2. It is the outer layer surface of skin i.e. epidermis acts like protecting shield against pathogens.
3. The epidermis layer lacks blood vessels and moisture and hence do not provide favourable environment for bacterial adhesion.
4. The hair present of on the surface of skin and inside the nose and ears play vital role in trapping the foreign particle and prevent entering it in to or body.

(2)Mucous membrane:-

• Mucous membrane of digestive, respiratory, urinary, reproductive system secrete mucous which trap microorganism out of the body.
• The mucous membranes also consist of cilia (hair like structure) which assist in trapping pathogens too.
• The sneezing and coughing allows to eject the trapped pathogens.

B) Chemical Barrier –

• The chemical barriers includes tears, sweat, saliva and breast milk.
• These barriers includes antimicrobial molecules.
• Tears consist of lysozyme that breaks the bacterial cell wall (like penicillin) causing bacterial death.
• In skin, the sebaceous glands secrete acids making it unfavourable for bacterial growth.
• In woman, the urinary and vaginal tract pH is also acidic inhibiting microbial growth.  In men, the seminal fluid consists of antimicrobial zinc and defensin protein.
• In stomach, the digestive juices are acidic in nature and contains antimicrobial protein called as pepsin.
• Glycerol monolaurate (GML), oligosaccharides, peptides are antimicrobial compounds present in the milk protect the new born babies from pathogen.  

C)Physiological barriers –

• Physiological barriers such as body temperature, pH and body secretions prevent growth of many invading pathogenic microorganisms.
• During the infection and inflammation, body temperature elevates which is known as  fever.
• Fever  inhibits the growth of many pathogens .
• Strong acidity of gastric juice in stomach (pH 1.2  to3.0 ) kills most of the ingested microorganisms.

D) Phagocytic barriers:-

• Phagocytes are specialized cells of our body that engulf and destroy the microbes and  protect the body.
• Most important phagocyte are macrophages , neutrophils and monocytes .For example Kupffur cells of liver.

E) Inflammatory barriers- 

• An  infection or tissue injury often causes redness, swelling , pain and production of heat that may result in fever. 
• Such localized  manifestation is called inflammatory response .
• This response is due to release of certain chemicals such as histamines and prostaglandins by damage mast cell of connective tissue and basophil of blood .
• This chemical dilate and take the  blood capillaries more permeable in the region of tissue injury .
• The vascular fluid comes out of the blood vessels. This fluid contains serum proteins which kills the bacteria.

Acquired Immunity:-

• Acquired immunity is the resistance that an individual acquires during its lifetime.
• It is found only in vertebrates and supplements the protection provided by innate immunity.
• This Immunity is acquired by activating immune cells and organs.

Characteristics –

• Specificity:- It is specific for each type of pathogen. It has the ability to differentiate between various foreign molecules.
• Diversity:- It has the ability to recognize vast variety of diverse and types of pathogens.
• Difference between self and non-self :-It is able to differentiate between own cells(self) and foreign cells or molecules (non-self).
• Memory:-When the immune system encounters a specific pathogen for the first time it generates immune response and eliminates the invader. The immune system retains the memory of this encounter. Second encounter with same pathogen stimulates immune response instantly.

The Acquired Immunity are of two types –  Active and passive

A) Acquired active immunity:-  

It is the resistance developed by an individual as a result of an antigenic stimulus of invading pathogens or by vaccine. It is further divided into two types –

Natural acquired active immunity:-

Immunity acquired due to infection is called as natural acquired active immunity. It is developed after the pathogen enters the body.

Artificial acquired active immunity:-

It is acquired artificially by vaccination. Vaccines consist of dead or alive but artificially weak pathogens or toxoids. They are introduced into the body and stimulates the formation of antibodies by the immune system.

B) Acquired passive immunity:-

It is acquired when ready-made antibodies are received by the body, it can be acquired either naturally or artificially.

Natural acquired passive immunity:-

Before birth maternal antibodies are transferred from mother to fetus through placenta while after birth the antibodies are transferred from mother to baby through milk. The antibody received by the child from mother remain in the body for short period of time.

Artificial acquired passive immunity:-

Immunity which is developed by injecting  previously prepared antibodies using serum from humans or animals is called artificial acquired passive immunity. Such immunity is provided to immune deficient people.

References –

Kindt, T., Goldsby, R., Osborne, B., Kuby, J. and Kuby, J., 2007. Kuby immunology. New York: W.H. Freeman.

https://bio.libretexts.org/Bookshelves/Human_Biology