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Immunity refers to the ability of an organism to recognize and defend itself against specific pathogens or antigens, meaning it is 'free from burden'.

The immune response is the third line of defense that involves the production of antibodies and the generation of specialized lymphocytes against specific antigens.

An antigen is a molecule from a pathogen or foreign organism that provokes a specific immune response.

The four main types of pathogens that cause disease are Viruses, Bacteria, Parasites, and Fungi.

The direct mechanisms of tissue damage by pathogens include:

1. Exotoxin production
2. Endotoxin
3. Direct cytopathic effect

The indirect mechanisms of tissue damage by pathogens include:

1. Immune complexes
2. Anti-host antibody
3. Cell-mediated immunity

Infectious agents associated with direct mechanisms include:

• Streptococcus pyogenes
• Staphylococcus aureus
• Corynebacterium diphtheriae
• Clostridium tetani
• Vibrio cholerae
• Escherichia coli
• Haemophilus influenzae
• Salmonella typhi
• Shigella
• Pseudomonas aeruginosa
• Yersinia pestis

Infectious agents associated with indirect mechanisms include:

• Hepatitis B virus
• Malaria
• Streptococcus pyogenes
• Treponema pallidum
• Mycobacterium tuberculosis
• Mycobacterium leprae
• Lymphocytic choriomeningitis virus
• Borrelia burgdorferi
• Schistosoma mansoni
• Herpes simplex virus

Diseases caused by pathogens utilizing direct mechanisms include:

• Tonsillitis, scarlet fever
• Boils, toxic shock syndrome, food poisoning
• Diphtheria
• Tetanus
• Cholera
• Gram-negative sepsis
• Meningitis, pneumonia
• Typhoid
• Bacillary dysentery
• Wound infection
• Plague
• Variola
• Chickenpox, shingles
• Hepatitis
• Polio virus
• Measles virus
• Influenza virus
• Herpes simplex virus
• Human herpes virus 8 (HHV8)

Diseases caused by pathogens utilizing indirect mechanisms include:

• Kidney disease
• Vascular deposits
• Glomerulonephritis
• Kidney damage in secondary syphilis
• Transient renal deposits
• Rheumatic fever
• Hemolytic anemia
• Tuberculosis
• Tuberculoid leprosy
• Aseptic meningitis
• Lyme arthritis
• Schistosomiasis
• Herpes stromal keratitis

The two main types of stem cells in the immune system are lymphoid stem cells and myeloid stem cells.

Lymphoid stem cells differentiate into B cells and T cells, which are collectively known as lymphocytes.

Myeloid stem cells differentiate into several types of cells including Monocytes, Basophils, Eosinophils, and Neutrophils.

Erythrocytes (red blood cells) and platelets are excluded from the classification of white blood cells; all other cells derived from stem cells are considered white blood cells.

• Dilates blood vessels and induces inflammation through the release of histamine and heparin.
• Recruits macrophages and neutrophils.
• Involved in wound healing and defense against pathogens.
• Responsible for allergic reactions.

Macrophages migrate from blood vessels into tissues and are phagocytic cells that consume foreign pathogens and cancer cells. They also stimulate the response of other immune cells.

Natural killer (NK) cells circulate in blood and migrate into tissues to kill tumor cells and virus-infected cells.

Dendritic cells present in epithelial tissue and tissues of the digestive tract migrate to lymph nodes upon activation. They present antigens on their surface (as antigen presenting cells/APCs), thereby triggering adaptive immunity.

Monocytes are stored in the spleen and move through blood vessels to infected tissues, where they differentiate into macrophages and dendritic cells in response to inflammation.

• First responders at the site of infection/trauma
• Represents 50-60% of all leukocytes
• Releases toxins that kill or inhibit bacteria and fungi
• Recruits other immune cells to the site of infection

• Responsible for defense against parasites
• Releases histamines that cause inflammation
• Can also be involved in allergic reactions

• Releases toxins that kill bacteria and parasites
• Can also cause tissue damage

• Produced and matures in the bone marrow, found in blood and lymph tissues
• Differentiates into plasma cells and memory cells
• Plasma cells are activated B cells that produce antibodies
• Memory cells are long-lived B cells that remember past antigen exposure

• Produced in the bone marrow and matures in the thymus, found in blood and lymph tissues
• Differentiates into T helper cells and cytotoxic T cells
• T helper cells enhance and promote the actions of other immune cells
• Cytotoxic T cells secrete lymphokines and destroy target cells

The components of the first line of defense include:

1. Skin
2. Mucous membranes
3. Secretions of skin and mucous membranes

The components of the second line of defense include:

1. Phagocytic white blood cells
2. Antimicrobial proteins
3. The inflammatory response

The components of the third line of defense include:

1. Lymphocytes
2. Antibodies

Innate immunity has no memory, resulting in the same response during the first encounter, while adaptive immunity has memory, leading to an amplified response upon subsequent encounters.

The skin consists of two layers:

1. Epidermis: A thin outer layer of epithelial tissue that contains Langerhans cells, dead cells, and keratin, which makes it waterproof.
2. Dermis: A thick inner layer of connective tissue that provides strength and elasticity to the skin.

Mucous membranes line the gastrointestinal, genitourinary, and respiratory tracts and consist of two layers:

1. Outer Epithelial Layer: Secretes mucus to maintain moist surfaces.
2. Inner Connective Layer: Provides support and structure to the epithelial layer.

The lacrimal apparatus contributes to the first line of defense by continually washing and blinking, which prevents microbes from settling on the eye surface.

Saliva washes microbes from teeth and mouth mucous membranes, helping to reduce the risk of infection.

Mucus is a thick secretion that traps many microbes, preventing them from entering the body.

Nose hairs are coated with mucus and filter dust, pollen, and microbes, helping to keep the respiratory system clear of pathogens.

Coughing and sneezing expel foreign objects from the respiratory tract, helping to prevent infection.

Urination cleanses the urethra, helping to remove microbes and prevent urinary tract infections.

Vaginal secretion removes microbes from the genital tract, helping to maintain a healthy environment and prevent infections.

Sebum is an oily substance produced by sebaceous glands that forms a protective layer over the skin. It contains unsaturated fatty acids which inhibit the growth of certain pathogenic bacteria and fungi.

Perspiration is produced by sweat glands and contains lysozyme and acids, which help to inhibit microbial growth on the skin.

Lysozyme is an enzyme that breaks down gram-positive cell walls. It is found in nasal secretions, saliva, and tears, contributing to the body's chemical defenses.

Gastric juices are a mixture of hydrochloric acid, enzymes, and mucus with a pH between 1.2 - 3. They destroy most microbes and toxins, serving as a chemical defense in the stomach.

Transferrins are iron-binding proteins in blood that inhibit bacterial growth by reducing the availability of iron, which is essential for bacterial proliferation.

Inflammation is triggered by tissue damage due to factors such as infection, heat, wounds, and other injuries.

The four major symptoms of inflammation are:

1. Redness
2. Pain
3. Heat
4. Swelling

Vasodilation is the increase in diameter of blood vessels. It is triggered by chemicals released by damaged cells, including histamine, kinins, prostaglandins, and leukotrienes.

Margination is the process in which phagocytes stick to the lining of blood vessels during inflammation, facilitating their movement towards the site of injury.

Diapedesis, also known as emigration, is the process by which phagocytes squeeze between endothelial cells of blood vessels to enter surrounding tissue during inflammation.

Phagocytes are attracted to the site of infection through chemotaxis and are responsible for destroying microbes, as well as dead and damaged host cells.

During the tissue repair stage, dead and damaged cells are replaced to restore the affected tissue.

1. Tissue Damage: Bacteria enter the skin and cause tissue damage.

2. Vasodilation and Increased Permeability: Blood vessels dilate, increasing permeability, leading to blood clot formation and abscess development.

3. Phagocyte Migration and Phagocytosis: Neutrophils and monocytes migrate from blood vessels to engulf bacteria.

4. Tissue Repair: Formation of a scab and regeneration of the epidermis and dermis.

Chemical mediators such as histamine, kinins, prostaglandins, and leukotrienes are released by damaged cells. They contribute to:

• Vasodilation: Widening of blood vessels to increase blood flow.
• Increased Permeability: Allowing immune cells to exit the bloodstream and reach the site of injury.

Margination refers to the process where phagocytes stick to the endothelium of blood vessels, while emigration is the subsequent squeezing of phagocytes between endothelial cells to exit the bloodstream. This is crucial for:

• Ensuring that immune cells reach the site of infection or injury.
• Facilitating phagocytosis of invading bacteria.

Tissue repair involves:

1. Formation of a Scab: Protects the underlying tissue during healing.
2. Regeneration of Epidermis: The outer skin layer is restored.
3. Regeneration of Dermis: The deeper skin layer is also repaired, restoring structural integrity.

The first step is chemotaxis and adherence of the microbe to the phagocyte.

During the ingestion phase, the phagocyte extends its plasma membrane to surround and engulf the microbe.

When the phagocyte engulfs the microbe, a phagosome is formed, which is a phagocytic vesicle containing the microbe.

When the phagosome fuses with a lysosome, it forms a phagolysosome, where digestive enzymes are present to break down the ingested microbe.

Digestive enzymes break down the ingested microbe within the phagolysosome during the digestion phase of phagocytosis.

The residual body contains indigestible material that remains after the digestion of the microbe.

The final step of phagocytosis is the discharge of waste materials from the phagocyte, which includes the residual body.

Mast cells are a type of immune cell found in tissues such as the skin and near blood vessels.

Mast cells are activated when an antigen binds to a specific type of antibody called IgE that is attached to receptors on the mast cell.

Activated mast cells release substances like histamine that contribute to inflammation.

Mast cells are important and involved in allergic responses.

The thymus is a glandular organ near the heart where T cells learn their jobs.

The bone marrow is a blood-producing tissue located inside certain bones, where blood stem cells give rise to all different types of blood cells.

The spleen serves as a filter for the blood, removing old and damaged red blood cells, as well as infectious agents, and activating lymphocytes.

Lymph nodes are small organs that filter out dead cells, antigens, and other substances to present to lymphocytes.

Lymphatic vessels collect fluid (lymph) that has leaked out from the blood into the tissues and return it to circulation.

T cells are responsible for cell-mediated immunity, which involves the direct attack of infected or cancerous cells.

T cells mature in the thymus gland.

B cells are responsible for antibody-mediated immunity, which involves the production of antibodies to neutralize pathogens.

B cells mature in the bone marrow.

Pluripotent stem cells are the origin of all blood cells, including immune cells. They have the ability to differentiate into various cell types, including lymphocyte stem cells, which further develop into B cells and T cells.

Lymphocyte stem cells can differentiate into B cells in the bone marrow or fetal liver, while T cells develop from lymphocyte stem cells that migrate to the thymus for maturation.

The thymus is responsible for the maturation of T cells from lymphocyte stem cells. It provides an environment for T cells to develop and undergo selection processes to ensure self-tolerance and functionality.

B cells are primarily found in lymphoid tissues such as lymph nodes, spleen, blood, and lymph, while T cells are also present in these tissues but are specifically matured in the thymus before entering circulation.

The primary site of development for T cells is the thymus.

B cells develop immunocompetence in the bone marrow.

After leaving the thymus or bone marrow, lymphocytes 'seed' secondary lymphoid organs, where they encounter antigens and become fully activated.

Secondary lymphoid organs are the sites of antigen challenge and the final differentiation of lymphocytes into fully activated B and T cells.

Mature immunocompetent lymphocytes recirculate continuously in the bloodstream and lymph, as well as throughout the lymphoid organs of the body.

T Lymphocytes are crucial in regulating the production of antibodies by B cells. They include:

1. Helper T Cells: Enhance and promote the action of other immune cells.
2. Cytotoxic/Killer T Cells: Have suppressive or cytotoxic effects.

T cells have surface proteins known as cluster determinants (CDs). These include:

• Helper T Cells: CD4 positive, which enhance and promote the action of other immune cells.
• Cytotoxic T Cells: CD8 positive, which exert suppressive or cytotoxic effects.

The key attributes of adaptive immunity are:

1. Antigenic specificity - Ability to recognize specific antigens.
2. Diversity - Capability to respond to a wide variety of antigens.
3. Immunologic memory - The ability to remember past infections for faster response upon re-exposure.
4. Self/ non-self recognition - Distinguishing between the body's own cells and foreign invaders.

Antigenic specificity refers to the immune system's ability to recognize and selectively eliminate specific foreign microorganisms or antigens, distinguishing subtle differences among them.

The immune system can recognize billions of unique structures on foreign antigens, showcasing its vast diversity in responding to various pathogens.

Immunological memory is the immune system's ability to remember an antigen after initial exposure, leading to a heightened state of reactivity upon subsequent exposures to the same antigen.

Self/non-self recognition is the immune system's capability to distinguish Self Antigens from Non-Self or Foreign Antigens, ensuring that the immune response is directed only against foreign antigens.

The two main divisions of the Adaptive Immune System are Humoral Immunity and Cell Mediated Immunity.

Humoral Immunity is provided by B Lymphocytes.

Cell Mediated Immunity is provided by T Lymphocytes.

Humoral Immunity can recognize Protein, Polysaccharide, Phospholipid, and Nucleic Acid Antigens.

Cell Mediated Immunity can recognize only Protein Antigens.

Humoral Immunity can act against soluble or free antigens.

Cell Mediated Immunity recognizes antigens presented by APCs with Class I or Class II MHC Molecules.

Humoral Immunity provides immunity to extracellular bacteria, viruses, and toxins.

Cell Mediated Immunity provides immunity to intracellular bacteria, viruses, fungi, and protozoa.

Humoral Immunity causes Type I, II, and III Hypersensitivity reactions.

Cell Mediated Immunity causes Type IV Hypersensitivity reactions.

The Major Histocompatibility Complex (MHC) displays cell-associated antigens for recognition by T cells, facilitating the immune response.

The two types of Major Histocompatibility Complex (MHC) are Class I MHC and Class II MHC.

The two classes of MHC are:

1. Class I MHC: Present on all nucleated cells.
2. Class II MHC: Present on macrophages, dendritic cells, and B cells.

Antigen-Presenting Cells (APCs) play a crucial role in the immune response by engulfing pathogens (like bacteria) and presenting their antigens on their surface using MHC molecules, which helps activate T cells.

MHC stands for Major Histocompatibility Complex. It is significant in the immune system as it is responsible for presenting antigen fragments on the surface of APCs, which is essential for T cell recognition and activation.

The cells classified as Antigen-Presenting Cells (APCs) include dendritic cells, macrophages, and B lymphocytes.

The Antigen enters the body.

APCs, such as Macrophages and Dendritic Cells, get activated and bind with Major Histocompatibility Complex II (MHC II) to form a complex.

Immature T Cells bind to the complex formed by APCs and MHC II with the help of T-Cell Receptors (TCR).

Activated CD4+ Cells release cytokines IL-2, IL-4, IL-5, and IL-6, which activate B Cells.

B Cells mature into Plasma Cells and Memory Cells.

Plasma Cells produce antibodies.

Memory Cells maintain immunological memory and protect the body from future exposure or infections from that particular antigen or microorganism.

After ingestion, the microbial antigen is partially digested by the APC, and the resulting fragments bind to MHC II molecules to form an MHC II/Ag complex on the surface of the APC, which is essential for T cell activation.

The Helper T cell binds to the MHC II/Ag complex presented by the macrophage, which is specific for the presented antigen. This binding is essential for the activation of the T cell and subsequent immune response.

The macrophage acts as an Antigen-Presenting Cell (APC) that engulfs microorganisms carrying T-dependent antigens, processes them, and presents antigen fragments on its surface along with Self (MHC) molecules.

The helper T cell's receptor binds to the antigen fragment presented by the macrophage, facilitating the activation of the T cell.

IL-2 is a growth factor that is released by the activated helper T cell to stimulate the appropriate B cell, promoting its activation and proliferation.

1. The macrophage engulfs a microorganism and presents antigen fragments.

2. The helper T cell binds to the antigen fragment on the macrophage.

3. The helper T cell releases IL-2 to activate the B cell.

T-dependent antigens require the assistance of helper T cells for B cell activation, leading to the production of antibodies against the specific antigen.

T helper cells interact with B cells, leading to the differentiation of B cells into Plasma cells and memory cells.

Plasma cells produce secreted antibodies, which are essential for targeting and neutralizing antigens.

B cells differentiate into Plasma cells and memory cells upon interaction with T helper cells.

1. Affinity: Memory cells produce antibodies that bind with greater affinity to their antigens than those produced in the initial response.

2. Response Time: The response time is much faster than the primary response.

3. Antibody Production: A greater number of antibodies are produced during the secondary response.

The humoral immune response begins with the antigen being engulfed by an antigen-presenting cell.

Helper T cells stimulate B cells and facilitate their differentiation into plasma cells that produce antibodies.

Plasma cells produce secreted antibodies that defend against extracellular pathogens.

Memory Helper T cells are crucial for a faster and more effective response during a second exposure to the same antigen, leading to the activation of Memory B cells and subsequent production of antibodies.

Upon re-exposure to the antigen, Memory B cells quickly differentiate into plasma cells, leading to a rapid production of antibodies.

Induced immunity is the use of antibodies to resist specific diseases. It can be classified into two main types: active immunity and passive immunity.

Active immunity is when antibodies are produced in the body. It can be acquired naturally, such as through catching a cold, or artificially, through vaccination, like the polio vaccination.

Natural active immunity occurs when pathogens enter the body naturally, stimulating the production of antibodies (e.g., catching a cold). Artificial active immunity occurs when pathogens are introduced into the body via a vaccine, prompting antibody production (e.g., polio vaccination).

Passive immunity involves the introduction of antibodies obtained from another organism into the body, as opposed to active immunity where the body produces its own antibodies.

The two types of passive immunity are natural and artificial. Natural passive immunity occurs when antibodies enter the body naturally, such as through the placenta to the fetus. Artificial passive immunity involves the injection of antibodies into the body, such as anti-tetanus injections.

Antibodies are proteins produced by B cells that identify and neutralize foreign objects such as bacteria and viruses.

Agglutination enhances phagocytosis and reduces the number of infectious units to be dealt with by clustering pathogens together.

Opsonization involves coating an antigen with antibody, which enhances the ability of phagocytes to recognize and engulf the pathogen.

Antibodies block the adhesion of bacteria and viruses to mucosa and also block the active site of toxins, preventing their harmful effects.

Antibodies can activate the complement system, which leads to the disruption of cells and attracts phagocytic and other defensive immune system cells.

Inflammation, triggered by the disruption of cells by complement or C-reactive protein, attracts phagocytic and other immune cells to the site of infection.

ADCC occurs when antibodies attached to target cells cause destruction by non-specific immune cells, enhancing the immune response against infected or abnormal cells.

Antibody binding blocks the adhesion of bacteria and viruses to mucosal surfaces and also blocks the active site of toxins, preventing their harmful effects.

Complement activation leads to cell lysis of bacteria, and the disruption of cells by complement or reactive proteins attracts phagocytic and other defensive immune system cells to the site of infection.

Antibody-dependent cell-mediated cytotoxicity occurs when antibodies attached to a target cell cause destruction by non-specific immune system cells, enhancing the immune response against infected or abnormal cells.

Inflammation is triggered by the binding of antibodies, which helps to recruit immune cells to the site of infection, facilitating a more effective immune response.

An antibody molecule consists of heavy chains and light chains connected by disulfide bonds. It also has an antigen-binding site at the top of the molecule.

The antigen-binding site in an antibody is responsible for recognizing and binding to specific antigens. This interaction is crucial for the immune response.

An epitope, also known as an antigenic determinant, is the specific part of an antigen that is recognized and bound by an antibody.

Any given antigen may have several epitopes.

Each epitope is recognized by a different antibody.

The five types of heavy chains are γ (gamma), δ (delta), α (alpha), μ (mu), and ε (epsilon).

There are two types of light chains in antibodies: K (kappa) and λ (lambda).

Heavy chains have 4-5 domains, while light chains have 2 domains.

An antibody molecule contains only one type of heavy chain and one type of light chain, which determines its classification into IgG, IgD, IgA, IgM, or IgE.

• Most common immunoglobulin, representing 75-80% of serum Ig.
• Only antibody capable of crossing the placenta, providing passive immunity to the fetus.
• Longest half-life of 23 days among all immunoglobulins.
• Activates complement.
• Stimulates chemotaxis.

• Acts as an antigen receptor on the surface of B cells.
• Activates basophils and mast cells to produce antimicrobial factors.

• Bound to the surface of mast cells and basophils.
• Destroys parasitic worms and participates in allergic reactions.

IgA is present in body secretions and provides protection against the proliferation of microorganisms in these fluids. It aids in defense against microbes and foreign molecules penetrating the body via cell linings of cavities.

IgA provides passive immunity to infants through the mother's breast milk, helping to protect them from infections during early life.

IgM is the first immunoglobulin class produced in a primary response to an antigen (Ag). It serves as the antigen receptor on B cells and is found on their surface alongside IgD. IgM has 10 antigen-binding sites, making it more effective at stimulating complement and facilitating opsonization through binding with Fc receptors on phagocytes.

IgM is characterized by its pentameric structure, consisting of five Y-shaped antibodies connected by a J chain in the center. This unique structure enhances its ability to bind antigens and activate the complement system.