Types of immunity

Our body has an influential immunity that saves us from various threats. These threats can come in the form of mechanical injuries, the entry of germs or the entry of other foreign particles like dust. Every day, we encounter a huge number of bacteria, viruses, fungi and other disease-causing agents but do not get harmed every time. This is due to our powerful immune system.

Immunity is mainly divided into two main parts i.e. innate immunity and adaptive or acquired immunity. 

Innate immunity

As the name recommends, it is the immunity that is found in the body since birth. Innate immunity is the body’s first intrinsic reaction to any intruder. 

Innate immunity is the non-specific type of immunity that attacks any pathogen in the same pattern. It has no memory means if it has targeted or attacked a pathogen whether it is bacteria, virus, fungi or any other harmful agent, it does not adapt it for any future response.

Innate immunity has further divisions which are described as the following:

External divisions

These include physical barriers such as skin, sweat, hair, mucous membranes, cilia, gastric secretions, etc.

Internal divisions

Internal divisions include chemicals, physiological responses and cellular responses. 

The physical barriers, chemicals and physiological responses are the first line of defense while cellular responses are the second line of defense. These two lines of defense perform their functions against pathogens. If both these lines of defense fail to fight against pathogens, the third line of defense comes into action and kills the pathogens.

Below is the detail of how the first line of defense protects our body from infectious and disease-causing agents. 

First Line of Defense

Skin

Our skin is the first component of the defensive system. Any pathogen trying to enter the body is first ceased by the skin.

Sebaceous glands in our skin release oils which have two important properties. First, these are made up of unsaturated fatty acids which serve as a poor energy source for microbes such as bacteria and viruses, etc. Second, the oil glands have pH ranges from 3-5 which makes an inhospitable environment for pathogens.

Our skin also secretes sweat which the environment salty for microbes and helps wash away any microbes that may be present on the skin. 

Hair

Our hair also traps dust particles and microbes and does not let them enter the body.

Mucous

The sticky viscous fluid of our organs is called mucous which helps to trap any pathogens trying to enter the body. This mucous contains defensins and lysozymes which are involved in invading pathogens and killing them so that our eyes, nasal cavity and oral cavities are protected. 

Cilia

If we tend to inhale something harmful, the little hairs present in our airways called cilia start beating and moving. So the harmful substances are caught in the mucous and removed by coughing or swallowing these substances into our stomach where they are destroyed by gastric secretions.

Gastric Secretions

Gastric secretions have pH ranges from 1-3 which can denature protein contents of pathogens.

Chemicals

Different types of chemicals in our body that provide us a defensive mechanism are around 30 complement proteins and cytokines such as interleukins, interferons, tumor necrosis factors, and some glycoproteins, etc. these chemicals not only support our innate immune system but also the adaptive immune system.

Physiological responses

The physiological responses related to our innate immunity are inflammation and fever. 

Inflammation: Inflammation is a damage that occurs to a vascularized tissue. Signs of inflammation include redness, heat, swelling, pain, etc. Inflammation is caused by the release of prostaglandins and histamines which are synthesized by cell membranes of damaged cells. Brady kinins are also released during tissue damage and they trigger nociceptors which cause pain in the damaged tissue.  These chemicals cause the dilation of damaged tissues and also enhance the vascular permeability so more immune cells and come to the damaged area and fix it. 

Fever: During fever, pyrogens stimulate heat in the body. Pyrogens reach the hypothalamus and stimulate it to raise the body temperature from 40 °C to 41°C. Fever makes our immune cells able to perform their functions in a better way and decreases the ability of pathogens to reproduce.

Second Line of Defense

Our second line of defense consists of various types of cells that are also a part of our innate immune system. These cells are discussed below;

Neutrophiles  

These are the most abundant type of white blood cells that target and engulf the pathogens mostly bacteria.

Natural killer cells

Also known as NK cells, they can directly target and kill pathogens. NK cells release perforins which make holes in pathogenic cells and host body fluid can enter the cells due to which the pathogenic cells can swallowed can burst. NK cells also release granzymes which the pathogens’ cells and trigger them to undergo apoptosis.

Macrophages

These may be fixed macrophages and wandering macrophages. 

Fixed macrophages such as histiocytes in our connective tissues, Kupffer cells in our liver, alveolar cells in our lungs and microglia in our nervous system can identify and engulf the pathogens.

Wandering macrophages float in the bloodstream. These are like monocytes which patrol in the bloodstream. When there is an infection in the body, these monocytes jump out of the bloodstream to go to the damaged area. They turn into macrophages, destroy the pathogens and present the protein parts or antigens of pathogens on their surfaces therefore monocytes are also called antigen-presenting cells.

Adaptive immunity

Adaptive or acquired immunity is the specific type of immunity that is not naturally present in the body rather it develops only at the time of pathogens’ exposure. 

Adaptive immunity is about learning to recognize particular pathogens and fighting against them. Adaptive immunity can differentiate between different pathogens. All pathogens in our body have different antigens present on their surfaces which are recognized by our immune cells. So, adaptive immunity individually works against every antigen.  

Once a specific pathogen is killed, adaptive immunity develops memory about it and if the same pathogen enters the body, the adaptive immunity immediately kills that pathogen without the need for any cellular activation mechanism.

In adaptive immunity, immunological memory is developed in four different ways which are described by two different types i.e. active immunity and passive immunity.

Active Immunity

Active immunity is developed when our body makes antibodies against certain pathogens naturally. This is called naturally acquired active immunity. 

The other way of developing immunological memory is by vaccination, when the body receives antibodies through artificial means for example in polio, we get exposed to an inactivated form of polio infection and the body creates antibodies against polio so we no longer get polio infection. This is called artificially acquired passive immunity.

Passive Immunity

In passive immunity, our body does not create antibodies itself rather it receives already made antibodies. This immunity can also be developed in natural and artificial ways.

Naturally acquired passive immunity is developed when a baby receives preformed antibodies within the breastmilk of his mother. These antibodies can show the best immunological response against pathogens.

 Artificially acquired passive immunity is where these antibodies are already produced in another person or another animal and these are injected into the infected person via the blood plasma or blood serum. For example, in tetanus, the preformed antibodies are injected into the patient's body to eliminate tetanus infection.

Third Line of Defense 

Adaptive immunity is the third line of defense which contains cellular and molecular responses. It means when the first and second lines of defense fail to fight against pathogens, then the third line of defense puts effort into fighting the battle against pathogens. 

The cells involved in the third line of defense include T cells and B cells and the molecules involved are antibodies.

There are two types of adaptive immunity which are Cell mediated immunity and humoral immunity.

Cell Mediated Immunity

The type of immunity in which cells are involved is called cell mediated immunity. It is controlled by T cells. T cells are named so because these are synthesized in the bone marrow and mature in the thymus. These are categorized as follows:

• T-killer or T cytotoxic cells: These cells directly kill the pathogens.
• T helper cells: T helper cells activate other cells such as dendritic cells and macrophages to stimulate immunity.
• T suppressor cells: These cells send signals for the inactivation of immunity when the pathogens are killed or eliminated from the body.
• T memory cells: T killer cells keep records of antigens for future reference and convert into T memory cells after killing the pathogens. If the same pathogen enters the body, the T memory cells immediately kill the pathogens.

Humoral Immunity

The Greek word humor refers to body fluid or plasma. Humoral immunity is named so because the cells involved in humoral immunity are synthesized from plasma cells. Humoral immunity involves the action of B cells and antibodies. Its main function is to provide immediate response against pathogens. 

When a large harmful molecule or a strong pathogen enters the body, the antibodies made by B cells recognize antigens present on its surface and send signals to macrophages to come and kill the pathogen.