Immunity refers to the ability of the body to protect itself from harmful microorganisms, viruses, and other foreign substances. The immune system is made up of a variety of cells, tissues, and organs that work together to identify and neutralize these threats. The two main types of immunity are innate immunity and adaptive immunity. Innate immunity is the body’s first line of defense and is present at birth, while adaptive immunity is acquired through exposure to antigens, such as bacteria and viruses, and is developed over time.

The immune system can be strengthened through healthy lifestyle choices, such as eating a balanced diet, getting regular exercise, and getting enough sleep. Vaccines can also help to boost immunity by introducing a small, harmless piece of a pathogen into the body, which allows the immune system to create a response to it.

However, sometimes the immune system can malfunction, leading to autoimmune diseases, where the immune system mistakenly attacks the body’s own cells, or immunodeficiency diseases, where the immune system is unable to properly defend the body against infections.

Innate Immunity

Innate immunity is the body’s first line of defense against pathogens and is present at birth. It is non-specific, meaning it does not target specific pathogens, but rather provides a general defense against a wide range of invaders. Innate immunity includes physical and chemical barriers, such as the skin and mucous membranes, as well as cellular components such as white blood cells, that work together to prevent pathogens from entering the body or to neutralize them if they do.

Physical Barriers:

• The skin serves as an effective barrier, preventing most pathogens from entering the body.
• Mucous membranes in the nose, mouth, and respiratory and digestive tracts trap pathogens and prevent them from entering the body.

Chemical Barriers:

• Cilia in the nose move mucus and trapped pathogens out of the body.
• Stomach acid kills many pathogens that enter the body through the mouth.
• Enzymes in the skin and mucous membranes also help to kill pathogens.

Cellular Components:

• White blood cells, such as neutrophils and macrophages, can engulf and destroy pathogens.
• Natural killer cells can identify and eliminate infected or cancerous cells.
• Innate immunity also includes the process of inflammation, which is the body’s response to injury or infection. This process helps to contain and eliminate pathogens by increasing blood flow to the affected area and recruiting immune cells to the site of infection.

Cytokine Barrier:

• Cytokine inhibits viral replication.
• Virus-infected cells secrete proteins called interferons which protect non-infected cells from a further viral infection is a cytokine barrier. 
• The cytokine barrier among these is interferon.

Adaptive Immunity

Adaptive immunity is the second line of defense against pathogens and is acquired through exposure to antigens, such as bacteria and viruses. It is specific, meaning it targets specific pathogens, and it develops over time. Adaptive immunity includes both humoral immunity and cellular immunity.

Humoral immunity:

• Involves the production of antibodies by B cells (also called B lymphocytes) that bind to and neutralize pathogens.
• Antibodies can also mark pathogens for destruction by other cells of the immune system.

Cellular immunity:

• Involves the activation and proliferation of T cells (also called T lymphocytes) that can directly attack and destroy infected cells.
• T cells can be divided into two main types: CD4+ T cells, also called helper T cells, and CD8+ T cells, also called cytotoxic T cells.

Adaptive immunity also involves the process of immunological memory, which is the ability of the immune system to remember and quickly respond to a pathogen that it has encountered before. This allows the body to mount a more rapid and effective response to a pathogen if it is encountered again in the future.

Adaptive immunity is considered as the second line of defense, it is more specific and takes time to develop, but once it is developed it allows the body to mount a more rapid and effective response to a pathogen if it is encountered again in the future.

Cells Of The Immune System

There are several types of cells that make up the immune system, which work together to identify and neutralize pathogens. Some of the main cells of the immune system include:

• White blood cells (also called leukocytes): These cells play a crucial role in the immune response and are divided into two main types: granulocytes and agranulocytes. Granulocytes, such as neutrophils and eosinophils, are important for engulfing and destroying pathogens. Agranulocytes, such as monocytes and lymphocytes, are also important for the immune response.

• T cells (also called T lymphocytes): These cells are a type of white blood cell that play a crucial role in cellular immunity. They can be divided into two main types: CD4+ T cells, also called helper T cells, and CD8+ T cells, also called cytotoxic T cells.
• B cells (also called B lymphocytes): These cells produce antibodies, which are proteins that bind to and neutralize pathogens
• Macrophages: These cells are a type of white blood cell that engulf and destroy pathogens.
• Natural Killer cells: These cells can identify and eliminate infected or cancerous cells.
• Dendritic cells: These cells present antigens to T cells to stimulate an immune response.
• Plasma cells: These cells produce antibodies to neutralize pathogens.

All these cells communicate and coordinate with each other to mount an effective response against invading pathogens.

Organ Involved in Adaptive Response

The main organs involved in the adaptive immune response are the lymphoid organs, which include:

• Thymus: This is where T cells mature and become able to recognize specific antigens.
• Spleen: This organ acts as a filter for the blood, trapping and removing pathogens, and also serves as a site for the production of antibodies.
• Lymph nodes: These small organs are located throughout the body and act as filters for the lymphatic system, trapping and removing pathogens. They also house immune cells that help to mount an immune response.
• Bone marrow: This is where all blood cells, including immune cells, are produced.

All these organs work together to generate, activate and maintain the immune cells (B and T cells) which are the key players in adaptive immunity. These immune cells migrate to different parts of the body to be ready for an immune response against a specific pathogen.