Human Health, Diseases & Immunity Notes | Study Biology Class 12 - NEET

NEET: Human Health, Diseases & Immunity Notes | Study Biology Class 12 - NEET

The document Human Health, Diseases & Immunity Notes | Study Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
All you need of NEET at this link: NEET

What is Health?

According to the World Health Organization (WHO), health is not only the absence of disease or illness. It is a state of an active and energetic condition of human beings including their physical, mental, and social well-being.

When people are healthy, they are more efficient at work. This increases productivity and brings economic prosperity. Health also increases the longevity of people and reduces infant and maternal mortality.
There are many factors that affect health, such as:

  • Poorly balanced diet.
  • Genetic Disorders.
  • Stress, and anxiety.
  • Infection from pathogens.
  • Intake of unhealthy and unhygienic food.
  • Lack of exercise and other physical activities.

To maintain good health, an individual should include a healthy and balanced diet, maintain personal hygiene along with regular exercise and other physical activities.

Biopsychosocial Model of HealthBiopsychosocial Model of Health

Everyone should be aware of the different types of diseases and their effects.

Try yourself:Which of the following does not affect health?
View Solution

What are Diseases?

A disease is an abnormal condition affecting a healthy living organism. It is broadly divided into infectious and non-infectious.

1. Infectious Diseases: These diseases are caused by the pathogens, such as bacteria, viruses, fungi, parasites and can be easily transmitted from one person to another, hence it is also known as a contagious or communicable disease. Common Cold, Tuberculosis, flu, ringworm, malaria are some examples of infectious diseases.

2. Non-infectious Diseases: Diseases that cannot be transmitted from one person to another are called non-infectious disease, it is also known as a non-communicable disease. These diseases can be either caused by genetic disorders, unhealthy diets, lack of physical activity, excessive use of tobacco, drugs, or alcohol, and few environmental factors.

Types of Diseases

The human body suffers from many diseases, it can be due to genetic defects, infections or an unhealthy lifestyle.
Diseases can be classified into two types:

  1. Congenital Diseases: Genetic defects present by birth. This may be due to gene mutation, chromosomal aberration, or environmental effects. Chromosomal and gene defects are transmitted to the next generation. E.g. Haemophilia, color blindness, Down syndrome, Turner’s syndrome, etc.
  2. Acquired Diseases: Diseases acquired during a lifetime.
    (i) Infectious or communicable disease: transmitted from one person to another
    (ii) Non-communicable disease: doesn’t spread by infection
    (iii) Deficiency disease: caused due to deficiency of an important nutrient, enzyme, or hormones, e.g. anemia, kwashiorkor, beriberi, diabetes, etc.
    (iv) Allergies: hypersensitivity to foreign substances, e.g. pollen, dust, mites, etc.

Common Infectious Diseases in Humans

  • The disease-causing organism is known as a pathogen, e.g. bacteria, virus, protozoan, fungi, worms
  • There are many ways by which a pathogen can enter our body

Human Health, Diseases & Immunity Notes | Study Biology Class 12 - NEET
Human Health, Diseases & Immunity Notes | Study Biology Class 12 - NEET

The Lifecycle of Plasmodium (Malarial Parasite)

  • Plasmodium enters the body by the bite of female Anopheles mosquitos
  • The infectious form is sporozoites, which comes from the saliva of female anopheles when they bite
  • It multiplies in the liver cells and then attacks RBCs resulting in RBCs rupture
  • Haemozoin, a toxic substance gets released
  • The gametocyte produced in the human blood gets transferred to the mosquito when it bites an infected person
  • Macro and micro gametocyte undergo fertilization, transformation and sporogenesis in the mosquito’s intestine and sporozoites are formed
  • Sporozoites migrate to the salivary gland of mosquito and the cycle is repeated
  • Plasmodium sp needs human and female Anopheles mosquito to complete their lifecycle

Life Cycle of MalariaLife Cycle of Malaria

AIDS (Acquired Immuno Deficiency Syndrome)

  • Caused by HIV (Human Immuno Deficiency Virus)
  • It is a retrovirus with RNA as its genome
  • The virus produces viral DNA in the host by the enzyme reverse transcriptase
  • The viral DNA gets incorporated into the host genome and multiple copies of the virus are produced
  • The virus attacks helper T-cells, where it replicates and multiplies, resulting in a marked decrease in the number of T lymphocytes
  • The infected person becomes immunodeficient after the virus attacks T- helper cells
  • AIDS patients become prone to various infections like mycobacterium, toxoplasma, fungal, and other viral infections
  • ELISA (Enzyme-Linked Immuno-Sorbent Assay) is a widely used diagnostic test for AIDS
  • AIDS may be transmitted by sexual intercourse, contaminated blood transfusion, using an infected syringe, or from mother to foetus through the placenta
  • NACO (National AIDS Control Organisation) works for the awareness and prevention of AIDS by educating people

HIV Virus Replication CycleHIV Virus Replication Cycle


  • Cancer is caused due to uncontrolled cell division leading to the formation of tumours
  • There is a breakdown of regulatory mechanism in the oncogenic transformation of normal cells
  • Cancerous cells lack contact inhibition property, which inhibits further growth of cell on contact with other cells
  • Benign tumours are non-invading and remain confined to their original location
  • Malignant tumours have invading ability and damage surrounding tissues
  • Metastasis: It is a property of malignant tumour when cells sloughed off from it reach distant sites and form a tumour in the various parts of the body
  • Cancer is caused due to DNA damage or genetic mutation resulting in the faulty regulation of the cell division
  • Cancer can also be caused due to the activation of proto-oncogenes present in normals cells under certain condition
  • Carcinogens: ionising radiation (e.g. X-rays, gamma rays), non-ionising radiation (UV rays), chemical agents (e.g. present in tobacco), viral oncogenes of oncogenic viruses
  • Cancer can be diagnosed by using a CT scan (computed tomography), MRI (magnetic resonance imaging), X-ray, PET scan (positron emission tomography) or by histopathological studies of tissue and blood
  • Cancer can also be diagnosed using molecular biology techniques to identify inherited susceptible genes for certain cancers
  • Antibodies against cancer antigens can also be used for diagnostic purpose
  • Cancer can be treated by surgery, transplantation, immunotherapy, radiation therapy
  • 𝛂-interferon act as biological response modifier, which activates the immune system to destroy tumour

What is Immunity?

Immunity is the ability of the body to defend itself against disease-causing organisms. 

  • Every day our body comes in contact with several pathogens, but only a few result in diseases. 
  • The reason is, our body has the ability to release antibodies against these pathogens and protects the body against diseases. This defense mechanism is called immunity.

Types of Immunity

  1. Innate Immunity or Natural or Non-specific Immunity
  2. Acquired Immunity or Adaptive Immunity.

Types of immunity Types of immunity 

1. Innate Immunity

  • This type of immunity is present in an organism by birth.
  • This is activated immediately when the pathogen attacks. Innate immunity includes certain barriers and defense mechanisms that keep foreign particles out of the body.
  • Innate immunity refers to the body’s defense system.
  • This immunity helps us by providing the natural resistance components including salivary enzymes, natural killer cells, intact skin and neutrophils, etc. which produce an initial response against the infections at birth prior to exposure to a pathogen or antigens.
  • It is a long-term immunity in which our body produces antibodies on its own. Our body has few natural barriers to prevent the entry of pathogens.

Types of Barriers

The four types of barriers are:

  • Physical barrier
    These include the skin, body hair, cilia, eyelashes, the respiratory tract, and the gastrointestinal tract. These form the first line of defense. The skin does more than providing us with fair or dark complexions. Our skin acts as a physical barrier to the entry of pathogens. The mucus coating in our nose and ear is a protective barrier that traps the pathogen before it gets inside.
  • Physiological barriers
    We know that our stomach uses hydrochloric acid to break down the food molecules. Due to such a strongly acidic environment, most of the germs that enter our body along with the food are killed before the further process is carried on.
    Saliva in our mouth and tears in our eyes also have the antibiotic property that does not allow the growth of pathogens even though they are exposed all day.
  • Cellular barriers
    In spite of the physical and physiological barriers, certain pathogens manage to enter our bodies. The cells involved in this barrier are leukocytes (WBC), neutrophils, lymphocytes, basophil, eosinophil, and monocytes. All these cells are all present in the blood and tissues.
  • Cytokine barriers
    The cells in our body are smarter than we give them credit for. For instance, in case a cell in our body experiences a virus invasion, it automatically secretes proteins called interferons which form a coating around the infected cell and prevent the cells around it from further infections.

Cells Involved In Innate Immunity

  • Phagocytes: These circulate through the body and look for any foreign substance. They engulf and destroy it defending the body against that pathogen.
  • Macrophages: These have the ability to move across the walls of the circulatory system. They release certain signals as cytokines to recruit other cells at the site of infections.
  • Mast Cells: These are important for healing wounds and defense against infections.
  • Neutrophils: These contain granules that are toxic in nature and kill any pathogen that comes in contact.
  • Eosinophils: These contain highly toxic proteins that kill any bacteria or parasite in contact.
  • Basophils: These attack multicellular parasites. Like the mast cells, these release histamine.
  • Natural Killer Cells: These stop the spread of infections by destroying the infected host cells.
  • Dendritic Cells: These are located in the tissues that are the points for initial infections. These cells sense the infection and send the message to the rest of the immune system by antigen presentation.

2. Acquired Immunity

  • Acquired immunity or adaptive immunity is the immunity that our body acquires or gains over time. Unlike innate immunity, this is not present by birth.
  • The ability of the immune system to adapt itself to disease and to generate pathogen-specific immunity is termed acquired immunity. It is also known as adaptive immunity.
  • An individual acquires immunity after birth, hence is called the acquired immunity.
  • It is specific and mediated by antibodies or lymphocytes which make the antigen harmless.
  • The main function of acquired immunity is to relieve the victim of the infectious disease and also prevent its attack in the future.
  • It mainly consists of an advanced lymphatic defense system that functions by recognizing the own body cells and not reacting to them.
  • The immune system of our body identifies the pathogens which have encountered in the past. It is mainly caused when a person comes in contact with the pathogen or its antigen.
  • Our body starts producing antibodies to engulf the pathogen and destroy its antigen.
  • When it encounters for the first time, it is called a primary response. Once a body gets used to these pathogens, antibodies are ready to attack them for the second time and are known as naturally acquired immunity.
  • The acquired immunity in our body has certain special features.

Types of acquired immunityTypes of acquired immunity

Features of Acquired Immunity

  • Specificity: Our body has the ability to differentiate between different types of pathogens, whether it is harmful or not, and devise ways to destroy them.
  • Diversity: Our body can detect vast varieties of pathogens, ranging from protozoa to viruses.
  • Differentiate between self and non-self: Our body has the unique ability to differentiate between its own cells and foreign cells. It immediately starts rejecting any foreign cell in the body.
  • Memory: Once our body encounters a pathogen, it activates the immune system to destroy it. It also remembers what antibodies were released in response to that pathogen, so that, the next time it enters, a similar procedure is followed by the body to eliminate it.

Cells Involved in Acquired Immunity

The acquired immunity involves two types of cells: B-cells and T-cells


  • They develop in the bone marrow.
  • These cells are activated on their encounter with foreign agents. These foreign particles act as foreign markers.
  • The B-cells immediately differentiate into plasma cells which produce antibodies specific to that foreign particle or so-called antigen.
  • These antibodies attach to the surface of the antigen/foreign agent.
  • These antibodies detect any antigen in the body and destroy it.
  • The immunity dependent on B-cells is called humoral immunity.


  • They originate in the bone marrow and develop in the thymus.
  • T-cells differentiate into helper cells, cytotoxic cells, and regulatory cells. These cells are released into the bloodstream.
  • When these cells are triggered by an antigen, helper T-cells release cytokines that act as messengers.
  • These cytokines initiate the differentiation of B-cells into plasma cells which release antibodies against the antigens.
  • The cytotoxic T-cells kill the cancer cells.
  • Regulatory T-cells regulate immune reactions.

Types of Acquired Immune Response

Humoral Immune Response

  • The antibodies produced by B-lymphocytes are present in the blood cells and they are transported all over the body. This is why it is called the humoral immune response as it consists of an antibody produced by the lymphocytes.
  • It depends upon the action of antibodies circulating in the body. When an antibody on a B-cell binds with an antigen, humoral immunity comes into play. The antigen is internalized by the B cell and presented on the helper T cell. This activates the B-cell.
  • The activated B cells grow and produce plasma cells.
  • These plasma cells release antibodies in the bloodstream. The memory B cells retain the information about the pathogen to prevent any disease caused by that pathogen in the near future.

Cell-mediated Immune Response

  • Cell-mediated immunity is initiated by the T helper cells.
  • The cytotoxic T cells eliminate the infected cells from the body by releasing toxins, thereby, promoting apoptosis or programmed cell death.
  • The T helper cells help to activate other immune cells. Cell-mediated immunity becomes clear in the case of transplant patients.
  • When any of our sense organs stop functioning, it can be transplanted to replace the malfunctioning organs. But it is not that simple with the immune response. It appears that T-lymphocytes are capable of recognizing whether tissue or an organ is from our body or foreign bodies. This is the reason why we cannot transplant and implant the organs into our body even if we find the donor with the same blood group because our body might reject the transplanted organ. 
  • The T-cells quickly recognize that the tissue or an organ is foreign and do not allow it to become a part of the body. This is why transplant receivers have to take immunosuppressant medication for the rest of their lives. This response is controlled by the T-lymphocytes.

Types of Acquired Immunity
a. Active Immunity

  • Active immunity involves the direct response to a foreign antigen within the body. In the case of the acquired or adaptive immune system, the body remembers the pathogens it has encountered in the past. This is a direct result of the active immune system.
  • Active immunity occurs when we are in contact with the pathogen or its antigen.
  • Antigens stand for antibody generator. It is with the help of antigens released by the pathogen that our body tackles the pathogen.
  • So what our body does is, it starts producing antibodies to attack the pathogen based on its antigen. When this happens for the first time, it is called a primary response. Once a body experiences a pathogen for the first time, it keeps a few of the antibodies that attacked the pathogen just in case it attacks for the second time. This is known as natural active immunity.

b. Passive Immunity

  • Passive immunity involves the immune response by the antibodies attained from outside the body. The primary response by the body to a pathogen it encounters for the first time is rather feeble, so the first encounter is always a little harsh on the body.
What if we could immunize everyone without the need for them ever getting sick?
  • Biotechnology has grown tremendously in the last decade or two and now we are capable of manufacturing antibodies for diseases. These ready-made antibodies protect the body even if the body hasn’t yet experienced a primary response.
  • While active immunity may protect us from a disease for a lifetime, passive immunity is more short-term.
  • Passive immunity develops immediately and our body could begin its attack on the pathogen right away.

There are two types of passive immunity:

  • Natural Passive Immunity
  • Artificial Passive Immunity


  • Sometimes the immune system attacks its own tissues and organs instead of the foreign agents. This is called autoimmunity. Type I diabetes is an example of an autoimmune disease.


  • A vaccine is made up of the antigens of the pathogen that cause the disease. For eg., the smallpox vaccine contains the antigens of the pathogen causing smallpox disease. When a person is vaccinated with the smallpox vaccine the antibody-producing cells are stimulated that produce smallpox antibodies. Thus, the body is protected against the disease occurring in the future.
  • Vaccinating pathogenic microbes into our body deliberately produces a similar response and is termed artificially acquired immunity.
  • Immunization is a process providing resistance to pathogenic microbes and other infectious diseases by the administration of a vaccine into the body. By immunization, it stimulates the body’s immune system to protect against subsequent infection or disease.

Try yourself:Which of the following is introduced during smallpox vaccination?
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Immune System in the Body

Major Immune components of lymphatic systemMajor Immune components of lymphatic system

  • The human immune system comprises lymphoid organs, cells and antibodies
  • Primary lymphoid organs: bone marrow and thymus. Here lymphocytes develop, mature, and differentiate to antigen-specific lymphocytes
  • Secondary lymphoid organs: spleen, lymph nodes, tonsils, Peyer’s patches in small intestine and appendix. These are the site for reaction with antigen and they become effector cells after the proliferation
  • The spleen acts as a filter of the blood. It contains lymphocytes and phagocytes and a large number of erythrocytes are present
  • Lymph nodes trap the antigens present in the lymph or tissue fluid
  • Mucosa-associated lymphoid tissue (MALT): the mucosal lining of the respiratory, urinary are digestive tract accounts for 50% of total lymphoid tissues present in the body

Drugs and Alcohol Abuse

  • Opioids, cannabinoids, and coca alkaloids are commonly abused drugs
  • There are opioid receptors present in our CNS and GI tract, where opioid drugs bind
  • Diacetylmorphine is commonly known as heroin or smack. It is extracted from the latex of the poppy plant Papaver somniferum. It is obtained by acetylation of morphine
  • Cannabinoids bind with the cannabinoid receptors present in the brain. They affect the cardiovascular system
  • Cannabinoids, e.g. marijuana, hashish, charas, ganja, etc. are obtained from the flower tops, leaves, resins of the plant Cannabis sativa
  • Cocaine or coca alkaloid is obtained from the plant Erythroxylum coca.
  • Cocaine acts by interfering with the transport of dopamine, a neurotransmitter
  • Atropa belladonna and Datura also have hallucinogenic properties
  • Sportspersons also take cannabinoids to enhance their performance, muscle relaxation, and reduce anxiety
  • Morphine is used as a sedative and pain killer
  • Barbiturates, amphetamines, benzodiazepines, etc. are used as a medicine for depression, insomnia, and other mental illness
  • Nicotine (alkaloid) present in tobacco stimulates the release of adrenalin and nor-adrenalin hormone by the adrenal gland. It increases heart rate and blood pressure
  • Smoking causes oxygen deficiency by increasing the concentration of carbon monoxide in the blood thereby decreasing the concentration of oxygen bound to hemoglobin
  • The excessive use of drugs and alcohol damages the nervous system and causes liver cirrhosis
  • The misuse of narcotic analgesic, anabolic steroids, diuretics to enhance performance and increase muscle strength is frequently done by sports person
  • Anabolic steroids induce masculinization and aggressiveness in females

Try yourself:What is Drug Addiction?
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The document Human Health, Diseases & Immunity Notes | Study Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
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