from Guyton and Hall textbook of medical physiology, 4th South Asia edition, write a 10-mark long answer on Immunity-define,classify;Cell Mediated Immunity;Humoral Immunity, Vaccination

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IMMUNITY - A 10-Mark Long Answer

(From Guyton and Hall Textbook of Medical Physiology, 4th South Asia Edition)

DEFINITION OF IMMUNITY

The human body has the ability to resist almost all types of organisms or toxins that tend to damage the tissues and organs. This overall capability is called immunity.

Immunity has two broad components:

Innate (Non-specific) Immunity - general protective mechanisms not directed at any specific pathogen
Acquired (Adaptive) Immunity - highly specific immunity developed after the body is first exposed to a particular bacterium, virus, or toxin

CLASSIFICATION OF IMMUNITY

I. Innate Immunity (Non-specific)

Innate immunity results from general processes rather than from processes directed at specific organisms. It includes:

Physical barriers - skin, digestive, urogenital and respiratory tract membranes, ciliated epithelium, and mucus that resist invasion
Physiological secretions - sweat, tears, saliva that inhibit microbial growth; gastric acid and digestive enzymes that destroy swallowed organisms
Phagocytosis by neutrophils, macrophages, and the tissue macrophage system
Chemical components in the blood:
- Lysozyme - a mucolytic polysaccharide that causes lysis of bacteria
- Basic polypeptides that inactivate gram-positive bacteria
- The complement complex (~20 proteins) that destroys bacteria
- Natural killer (NK) lymphocytes that recognize and destroy foreign cells, tumor cells, and infected cells

II. Acquired (Adaptive) Immunity

Acquired immunity is the ability to develop extremely powerful specific immunity against individual invading agents. It involves antibodies and/or activated lymphocytes that attack and destroy the specific organism or toxin.

Acquired immunity is further classified into two types:

Feature	Humoral Immunity	Cell-Mediated Immunity
Mediated by	B lymphocytes	T lymphocytes
Effectors	Circulating antibodies	Activated T cells
Also called	B-cell immunity	T-cell immunity
Target	Bacteria, toxins, viruses	Intracellular pathogens, tumor cells, transplants

ANTIGENS

Both types of acquired immunity are initiated by antigens. Antigens are specific chemical compounds (usually proteins or large polysaccharides) that are different from all compounds of the host and that initiate acquired immunity. For a substance to be antigenic:

It usually must have a molecular weight of 8,000 or more
It must have regularly recurring molecular groups on its surface called epitopes

LYMPHOCYTES - THE BASIS OF ACQUIRED IMMUNITY

Lymphocytes are responsible for acquired immunity. The body contains about 1 trillion lymphocytes stored mainly in the lymph nodes, spleen, thymus, submucosal areas of the GI tract, and bone marrow.

T lymphocytes (thymus-derived) - mature in the thymus
B lymphocytes (bone marrow-derived) - mature in the bone marrow

Both types of lymphocytes are derived from pluripotent hematopoietic stem cells in the embryonic yolk sac, liver, and bone marrow.

Lymphocyte Clones: Each lymphocyte is already pre-programmed to react against a specific antigen. Millions of different lymphocyte clones exist, each capable of responding to one specific antigen. When an antigen enters the body, it selectively activates only those clones specific to it. The activated lymphocytes then proliferate greatly, producing memory cells (for future rapid response) and effector cells.

CELL-MEDIATED IMMUNITY (T-LYMPHOCYTE SYSTEM)

When lymphocytes are activated by an antigen (presented by macrophages bearing the antigen on their surfaces along with an MHC molecule), the T lymphocytes form activated T cells. These are released into lymph and circulate throughout the body.

Types of T Cells and Their Functions

1. T-Helper Cells (CD4+ cells) - the most numerous These are the dominant regulatory cells of the immune system. They:

Secrete lymphokines (cytokines) that regulate the activity of almost all other immune cells
Help activate cytotoxic T cells and B cells
Stimulate macrophage activity
Stimulate proliferation of all T and B cell clones
Important lymphokines secreted include: interleukins (IL-2 through IL-6), interferon-γ, colony-stimulating factors, and tumor necrosis factor

2. Cytotoxic T Cells (CD8+ cells) - "Killer Cells" These are direct attackers of foreign cells. They:

Attack virus-infected cells, tumor cells, transplanted tissue cells
Cause lysis of cells by secreting perforins (hole-forming proteins) and granzymes (enzymes that trigger apoptosis)
Release cytotoxic substances directly into the target cell
A single cytotoxic T cell can kill multiple target cells sequentially

3. Regulatory T Cells (Tregs) These suppress immune responses and help prevent autoimmunity. They release inhibitory cytokines (e.g., IL-10, TGF-β) that suppress activation of other T cells and B cells.

Mechanism of Cell-Mediated Immunity

When a macrophage presents antigen to a T-helper cell (via MHC class II), IL-1 is released, which activates T-helper cells to produce IL-2. IL-2 further stimulates T-cell proliferation. Cytotoxic T cells require direct antigen presentation via MHC class I. The activated T cells attack and destroy the specific target cells bearing the antigen.

HUMORAL IMMUNITY (B-LYMPHOCYTE SYSTEM AND ANTIBODIES)

When antigens come into contact with B lymphocytes, the B cells rapidly enlarge and differentiate into plasma cells, which then produce antibodies. These antibodies circulate in the blood and attack the invading antigen. The lymph nodes draining the site of invasion are the main sites of this response.

Antibody Structure

Each antibody molecule is a gamma globulin with a molecular weight of 160,000 to 900,000. It consists of:

Two heavy chains (long polypeptides)
Two light chains (short polypeptides)
Connected by disulfide bonds
Each molecule has two antigen-binding sites (Fab regions) located at the two ends of the "Y" shape, formed by variable regions

There are five classes of immunoglobulins:

IgG - most abundant; secondary response; crosses placenta; provides passive immunity to newborn
IgA - secretory immunity; present in body secretions
IgM - first antibody produced in primary response; 10 antigen-binding sites
IgD - surface receptor on B cells; triggers B-cell activation
IgE - involved in allergic reactions; binds to mast cells and basophils

Primary vs. Secondary Response

Primary response: On first exposure, IgM is produced first, then IgG, after a lag of 1-2 weeks
Secondary (anamnestic) response: On re-exposure, due to memory B cells, a much faster, stronger, and more prolonged IgG response occurs (within 2-3 days, lasting months to years)

Mechanisms of Action of Antibodies

Antibodies work by the following mechanisms:

Agglutination - antibodies cause foreign particles bearing multiple antigens to clump together
Precipitation - the antigen-antibody complex precipitates out of solution
Neutralization - antibodies cover the toxic sites on antigens, rendering them harmless
Lysis - potent antibodies occasionally directly rupture the membranes of cellular agents

Most protection occurs through the amplifying effects of the complement system.

Complement System

The complement system consists of ~20 proteins (C1 through C9, B and D) that circulate in the blood as inactive precursors. It is activated through the classical pathway (antigen-antibody reaction) and the alternative/lectin pathways.

Effects of complement activation:

Opsonization - C3b markedly enhances phagocytosis by neutrophils and macrophages (hundredfold)
Lysis (cytolysis) - the Membrane Attack Complex (MAC) = C5b-6-7-8-9 inserts into the lipid bilayer of pathogens, forming pores that cause osmotic rupture
Agglutination - complement products change surface of invading organisms, causing them to clump
Neutralization of viruses - complement enzymes attack virus structures rendering them non-virulent
Chemotaxis - Fragment C5a attracts neutrophils and macrophages to the site
Mast cell and basophil activation - C3a, C4a, C5a release histamine and heparin, promoting inflammation

VACCINATION (IMMUNIZATION BY INJECTION OF ANTIGENS)

Vaccination is the process of immunization - inducing acquired immunity artificially before exposure to a pathogen.

Active Immunization (Vaccination)

Vaccination works by injecting antigens (in the form of attenuated or killed organisms, toxoids, or subunit antigens) into the body. This activates the immune system to produce both antibody-forming clones of B lymphocytes and sensitized T lymphocyte clones, which then persist as memory cells for months to years.

On subsequent exposure to the same antigen, the secondary (anamnestic) immune response rapidly eliminates the pathogen before disease can develop.

Key principle: Vaccines mimic natural infection without causing disease, creating immunological memory.

Types of vaccines:

Live attenuated vaccines (e.g., MMR, oral polio, BCG) - weakened organisms; strongest immunity
Killed/inactivated vaccines (e.g., IPV, hepatitis A) - killed organisms
Toxoids (e.g., tetanus, diphtheria) - inactivated bacterial toxins used as antigens
Subunit/recombinant vaccines (e.g., hepatitis B, HPV)

Passive Immunity

Passive immunity is achieved by injecting preformed antibodies from another person or animal into a recipient. Examples include:

Injecting gamma globulin from a recovered patient
IgG crosses the placenta to provide passive immunity to the fetus and newborn
IgA is transferred via breast milk

Passive immunity provides immediate but temporary protection (weeks to months), as the antibodies are gradually catabolized and no memory cells are created.

Clinical Importance

Acquired immunity can provide protection against certain toxins (e.g., botulinum, tetanus) in doses as high as 100,000 times the lethal dose without immunity. This remarkable degree of protection underlies the rationale for immunization programs worldwide.

TOLERANCE - PREVENTION OF SELF-ATTACK

The acquired immune system normally does not attack the body's own tissues. This self-tolerance is established during lymphocyte maturation:

In the thymus: T lymphocytes that react against self-antigens are destroyed (clonal deletion)
In the bone marrow: Self-reactive B lymphocytes are similarly eliminated

Failure of tolerance leads to autoimmune diseases.

SUMMARY DIAGRAM

IMMUNITY
├── INNATE (Non-specific)
│   ├── Physical barriers
│   ├── Phagocytosis
│   ├── Complement (alternative/lectin pathway)
│   └── NK cells
└── ACQUIRED (Specific/Adaptive)
    ├── Humoral (B cells → Antibodies → Complement → Opsonization, Lysis, etc.)
    └── Cell-Mediated (T cells → T-helper, Cytotoxic T, Regulatory T cells)

All content sourced from: Guyton and Hall Textbook of Medical Physiology, 4th South Asia Edition - Chapter 35: Resistance of the Body to Infection: II. Immunity and Allergy.

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