Immunity And Immune System: Definition, Difference, Examples

What is Immunity?

The immune system is the body’s main defence. It fight against pathogenic agents, which include bacteria, viruses, and other invaders.This system includes immune cells, tissues, and organs. It works to detect and destroy invaders. Immunity can be natural or vaccine-induced. Natural immunity comes from past infections. Vaccine immunity builds protection without illness. Some of the major points about immunity are discussed below:

• The immune system includes various cells (like white blood cells) and organs (such as lymph nodes, spleen, and thymus) that work together to detect and destroy pathogens.

• Immune cells are produced by B-cells, antibodies are proteins that specifically target and neutralize foreign agents like bacteria and viruses, marking them for destruction.

• Vaccination help the immune system to develop adaptive immunity by introducing harmless antigens.

• This allows the body to recognize and respond to future infections more effectively based on the immune system's memory cells.

Immune System

The immune system protects the body from pathogens. It fights bacteria, viruses, and fungi. It stops germs from causing infection. There are two main types of immunity.

1. Innate immunity

2. Adaptive immunity

Immunology is a branch of medicine. It studies the immune system and how it works. It looks at how the body fights foreign particles. This includes bacteria, viruses, and other pathogens. It covers both innate and acquired immunity. Immunology also studies what happens when the system goes wrong. Immunologists develop vaccines and immunotherapies to improve health.

Types of Immunity: Innate and Adaptive

Immunity is classified into two main types: innate immunity and acquired immunity. Innate immunity is immediately available at birth and provides primary protection, while acquired immunity is developed as the organism responds to various pathogens as it grows. It is important to understand the difference between innate immunity and acquired immunity.

Innate Immunity

Various cells are involved in innate immunity:-

• Phagocytes: Engulf and digest pathogens.

• Macrophages: Destroy pathogens, present antigens, and initiate adaptive immunity.

• Neutrophils: First responders that immediately eliminate pathogens.

• Natural Killer (NK) Cells: Targets and destroys infected or malignant cells.

• Mast Cells: Release histamine and are involved in allergies and defence against parasites.

• Eosinophils: Defend against parasites which participate in allergic responses.

• Basophils: Contribute to inflammation and allergic reactions.

• Dendritic Cells: Present antigens that help connect innate and adaptive immunity.

Acquired Immunity

Various cells and processes are involved in acquired immunity:

• B Cells: Produce antibodies that target specific pathogens. Plasma Cells: Formed from B cells and secrete large amounts of antibodies.

• Memory B Cells: Remember past infections and respond faster on re-exposure.

• T Cells: Help kill infected cells and support other immune responses. Helper T Cells (CD4⁺): Activate B cells and other immune cells.

• Cytotoxic T Cells (CD8⁺): Directly kill infected or abnormal cells.

• Memory T Cells: Provide long-lasting immunity by remembering past pathogens.

• Antigen-Presenting Cells (APCs): Display antigens to T cells to activate adaptive response.

Types of Barriers in Innate Immunity

The innate immune system is the body’s first defense. It protects against bacteria, viruses, and fungi. It acts fast and is always ready. Each barrier helps stop germs from entering the body.

Types of Acquired Immunity

The immune system protects the body from diseases. It uses two main types of acquired immunity: active immunity and passive immunity. In active immunity, the body produces its own antibodies after infection or vaccination.Passive immunity helps when fast immune response is needed.

Cells involved in acquired immunity

Primarily, 2 kinds of cells are involved in acquired immunity:

B Cells

B cells are other large mononuclear leukocytes that are involved in the acquired immunity besides actively participating in the humoral immunity. They produce substances that form a lock-key system with particular antigens that are present on the pathogen.

On stimulation, the B cells differentiate through proliferation to plasma cells or memory B cells and start producing antibodies. B cells also differentiate into memory B cells to give the organism long-term immunity.

T Cells

T lymphocytes are significant leukocytes that are involved in acquired immunity, especially in cell-mediated immunity. Effector T cells, also known as cytotoxic T cells, CD8 T cells are involved in killing infected or cancerous cells by apoptosis.

CD4+ T cells or helper T cells mobilize immune responses through the active identification of antigens and releasing substances known as cytokines which stimulate other immune cells. Memory T cells are alive after the first immune response and can give quick and stronger responses when encountering similar antigens again.

Different Immune Response Mechanisms

The immune system fights infections using two main methods — humoral immunity and cell-mediated immunity. These two immune responses differ in how they detect and eliminate pathogens. The difference between humoral and cell-mediated immunity lies in the type of immune cells involved and the way they target invaders.:

• Humoral Immune Response

B cells are special white blood cells. They find and attach to harmful germs or toxins. After this, B cells change into plasma cells. Plasma cells make proteins called antibodies. Antibodies travel in the blood and lymph. They stick to germs and mark them. This helps kill the germs or remove them from the body.

• Cell-Mediated Immune Response

T cells are another type of white blood cell. They work in the body tissues, not in the blood. T cells help fight viruses and bacteria inside cells. Cytotoxic T cells (CD8+) kill infected or cancer cells. They do this by making the cells die (apoptosis). Helper T cells (CD4+) guide the immune system. They release chemicals called cytokines. Cytokines help other immune cells work better.

Diagram Representing Lymphoid Organs

Lymphoid Organs

Human lymphatic system consists of those tissues that contribute to the formation, differentiation, and activation of immune cells.

Primary Lymphoid Organs

• Bone Marrow: More specifically, bone marrow is the part of the human body that is responsible for producing all types of blood cells, the Erythroctes (red blood cells) and the white ones included.

• Thymus: It is in the thymus that T cells develop to be able to respond to a given antigen.

Secondary Lymphoid Organs

• Spleen: The spleen is involved with blood filtering as well as leading the immune response by presenting the antigens to the cells involved in the immune response.

• Lymph Nodes: Lymph nodes act as a filter for the lymph as well as a place where containing immune cells harmonise as well as plan on how best to deal with the pathogen.

Tertiary lymphoid organs

Tertiary lymphoid organs are observed in the zones of chronic inflammation and are involved in local immune processes.

How the Immune System Works

The working mechanism of the immune system is explained below:

• Recognition of Pathogens: Organisms of disease-causing agents are identified by the immune cells through specific antigens located on the outer membrane of the pathogens.

• Response to Pathogens: When it senses a pathogen, the immune system initiates an attack to get rid of it. This also involves stimulation of the immune cells and resources, the release of antibodies, as well as inflammation.

• Primary Immune response: This is the first time the immune system comes across the pathogen, causing activation and multiplication of immune cells.

• Secondary Immune response: The secondary immune response is usually faster and more efficient due to the point that it contains memory cells that are acquainted with the pathogen.

• Role of Memory Cells in Acquired Immunity: Some cells in the body keep information about the pathogens that have been encountered before to ensure a quick and efficient response.

Disorders of the Immune System

The immune system protects our body from harmful germs. But sometimes, it doesn't work properly. It may attack the body’s own cells or fail to fight infections. These problems are called immune system disorders. Major disorders of the Immune system are discussed below:

• Autoimmune diseases: Autoimmune diseases are conditions where the immune system turns against the body and starts attacking the body’s tissues. Some basic examples are:

• Rheumatoid Arthritis: The immune system targets the joints of the body causing inflammation.

• Type 1 Diabetes: The immune system of the body malfunctions and goes after the insulin-producing cells in the pancreas.

• Immunodeficiency disorders: Immunodeficiency disorders are those in which the body’s immune system is compromised in some manner.

• HIV/AIDS: HIV infects those cells and destroys the body’s defence system, which is the immune system, resulting in AIDS.

• SCID (Severe Combined Immunodeficiency): An inherited disease in which the body’s ability to fight infections is significantly impaired.

• Allergies and Hypersensitivities: Allergy is a condition where the immune system of the body behaves abnormally, resulting in response to substances that do not pose any threat to the body, including pollen and certain foods, and this results in some symptoms such as sneezing, itching and swelling of certain parts of the body.

MCQs on Immune System

Question: In ‘T-lymphocytes’, the letter ‘T’ refers to

1. Thalamus

2. Tonsil

3. Thymus

4. Thyroid

Answer: The T-lymphocytes, named for the thymus, are very important in the cell-mediated immune system. These immature lymphocytes start in the bone marrow. They move through the bloodstream to be in the thymus where they mature into functional T-lymphocytes. Once in the thymus, they receive selection processes that verify their ability to react upon antigens properly and avoid reacting against themselves. For a good, strong immune response, this matured state should occur.

Hence the correct answer is Option 3) Thymus

Question: The type of immunity in which a person’s own body produces an immune response in terms of antibodies against some kind of infection or vaccination is said to be

1. Passive immunity

2. Humoral immunity

3. Innate immunity

4. Active immunity

Answer: The type of immunity in question is active immunity.

Characteristics of Active Immunity:
- It involves the body's production of antibodies and memory cells as a response to an infection or vaccine.
- It is typically long-lasting due to the immune system's ability to remember pathogens for future defense.

Active Immunity can be initiated by:
- Natural infection: Exposure to a live pathogen (virus or bacterium) prompts the immune system to create a specific response.
- Vaccination: Introduction of a weakened or killed pathogen, or its components (e.g., proteins), which induces the creation of antibodies and memory cells without causing illness.

Hence, the correct answer is option 4) Active immunity.

Also Read:

• Protozoan Disease

• Viral Diseases

• Bacterial Diseases