Comprehensive Study Notes on Specific Immune Response and Adaptive Immunity

Leukocytes: White Blood Cells Classification and Function

Leukocytes are broadly classified into granulocytes and agranulocytes. Granulocytes include Neutrophils (PMNs), which represent $60-70\%$ of leukocytes and function primarily in phagocytosis. Basophils, comprising $0.5-1\%$ of the total, are responsible for the production of histamine. Eosinophils, making up $2-4\%$ of leukocytes, produce toxic proteins directed against certain parasites and engage in some phagocytosis.

Agranulocytes consist of Monocytes, Dendritic cells, and Lymphocytes. Monocytes comprise $3-8\%$ of total leukocytes and perform phagocytosis once they mature into macrophages. Dendritic cells function in phagocytosis and the initiation of adaptive immune responses. Lymphocytes, which represent $20-25\%$ of leukocytes, are subdivided into three types: Natural killer (NK) cells, which destroy target cells via cytolysis and apoptosis; T cells, responsible for cell-mediated immunity; and B cells, which produce antibodies.

Types of Adaptive Immunity

Adaptive immunity is classified into naturally acquired and artificially acquired categories, further divided into active and passive forms. Naturally acquired active immunity occurs when antigens enter the body naturally, inducing the production of antibodies and specialized lymphocytes. Naturally acquired passive immunity involves the transfer of antibodies from a mother to a fetus via the placenta or to an infant via mother's milk.

Artificially acquired active immunity is achieved when antigens are introduced through vaccines, prompting the body to produce antibodies and specialized lymphocytes. Artificially acquired passive immunity involves the introduction of preformed antibodies found in immune serum via injection.

Lymphocytes and Antigen-Binding Proteins

Each B cell or T cell produces a unique protein that interacts with a single type of antigen, granting these proteins high specificity. The antigen-binding proteins on B cells are membrane-bound antibodies known as B cell receptors (BCRs). B lymphocytes specialize in producing antibodies that protect against extracellular antigens, conferring antibody-mediated (humoral) immunity. T lymphocytes express surface receptor proteins that defend against intracellular pathogens, such as viruses and certain bacteria, conferring cell-mediated (cellular) immunity.

Humoral Immunity and Antibody Structure

Humoral immunity involves soluble proteins called antibodies (Abs) or immunoglobulins (Ig), which move freely to provide protection against foreign bodies. These are produced by B cells differentiating in the bone marrow. Secreted antibodies are produced by plasma cells (B cells) in secondary lymphoid tissues, specifically the spleen. Membrane-bound antibodies on B cell surfaces act as antigen-specific receptors for antigen-presenting cells (APC).

An antibody molecule consists of two heavy (H) chains ($440$ amino acids each) and two light (L) chains ($220$ amino acids each). Structure-wise, an antibody is divided into the Fab (fragment antigen binding) region, where variable domains determine specificity and join with antigens, and the Fc (crystalline fragment) region, which influences immune responses like cell lysis by combining with certain proteins. The Fc region mediates effector functions such as antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC).

Molecular Detail of Immunoglobulins

Disulfide bonds bind heavy chains to light chains and heavy chains to each other, forming loops within the chains at a constant range of approximately $100$ amino acids. The light chain contains $2$ domains (one variable, one constant), while the heavy chain contains $4$ or $5$ domains (one variable, three or four constant). The variant part at the amino terminus differs significantly between molecules. The hypervariable parts on both H and L chains determine the combination with the antigen, forming a complementary region to the antigen epitope known as the complementarity-determining regions: CDR1, CDR2, and CDR3. Diversity in these CDRs provides different specificities and functions.

A hinge region provides flexibility between the two Fab arms, consisting of a short chain of amino acid residues (cystine and proline) located between CH1 and CH2. Cysteine allows for interchain disulfide bond formation, while proline inhibits rolling into a globular structure. Glycan molecules also assist this structure.

The Five Classes of Antibodies

There are five classes of antibodies in most vertebrates based on their heavy chains: $\mu$ (IgM), $\gamma$ (IgG), $\alpha$ (IgA), $\epsilon$ (IgE), and $\delta$ (IgD).

IgM is a pentamer with $10$ antigen-binding sites and a molecular weight of $900,000\,\text{Daltons}$. It makes up $6\%$ of serum antibodies, does not cross the placenta, fixes complement, and is the first antibody produced in primary responses. Its monomer form serves as a B cell receptor.

IgG is a monomer with $2$ antigen-binding sites and a molecular weight of $150,000\,\text{Daltons}$. It constitutes $80\%$ of serum antibodies, crosses the placenta, fixes complement, and binds to phagocytes. It is the main blood antibody of secondary responses, neutralizes toxins, and aids opsonization.

Secretory IgA is a dimer with $4$ antigen-binding sites and a molecular weight of $385,000\,\text{Daltons}$. It makes up $13\%$ of serum antibodies and is secreted into mucus, tears, saliva, and colostrum.

IgE is a monomer with $2$ antigen-binding sites and a molecular weight of $200,000\,\text{Daltons}$. It constitutes $0.002\%$ of serum antibodies and binds to mast cells and basophils, acting as the antibody for allergy and antiparasitic activity.

IgD is a monomer with $2$ antigen-binding sites and a molecular weight of $180,000\,\text{Daltons}$. It makes up $1\%$ of serum antibodies and serves as a B cell receptor.

Antigens, Epitopes, and Haptens

An antigen (Ag) or immunogen is a substance that triggers an acquired immune response. While all immunogens are antigens, not all antigens are immunogens. An antigen is capable of interacting specifically with immune components, whereas an immunogen is specifically capable of influencing an immune response.

Immunodominant epitopes are groups on the surface of antigens that most strongly influence the immune response. A paratope is the complementary part of the antibody molecule that joins the epitope. Haptens are small molecules with low molecular weight that can only act as antigens if bound to a larger carrier protein. An example is penicillin, which can act as a hapten and provoke an allergic reaction when bound to a protein.

Antibody Functions Against Pathogens

Antibodies utilize several mechanisms to protect the host. Neutralization occurs when antibodies block the binding of toxins or viruses to cell-surface receptors. This includes blocking bacterial adhesins to prevent colonization. Opsonization involves coating the antigen with antibodies to enhance phagocytosis by cells like macrophages. Agglutination reduces the number of infectious units by clumping them together.

Complement activation (Classical Pathway) triggers antibacterial activity through the formation of a membrane attack complex that lyses the pathogen. It also produces anaphylatoxins to recruit phagocytic cells and induce inflammation. Antibody-dependent cell-mediated cytotoxicity (ADCC) involves antibodies attaching to a large target cell (like a parasite), causing its destruction by macrophages, eosinophils, or NK cells using perforin and lytic enzymes.

Humoral Immune Response Kinetics

The primary response occurs when an antigen is first recognized by host B cells. After a lag period of approximately $5\,\text{days}$, antibodies appear in the blood. IgM is the first antibody produced at low titers. B cells divide to form plasma cells and memory cells, which remain in lymphoid tissues.

The secondary response occurs upon re-exposure to the same antigen. This response is much faster and stronger due to immune memory cells (clones). The antibody class switches rapidly to IgG, which is produced in high titers. Specificity ensures that immune cells react only with individual antigens, while memory ensures a heightened response upon second contact.

B Cell Selection and Tolerance

B cell selection occurs when BCRs on a specific B cell clone interact with their corresponding antigen, leading to clonal expansion (proliferation and differentiation). To prevent the destruction of host tissues, the immune system employs clonal deletion, destroying B and T cells that react to self-antigens. Tolerance refers to the ability to recognize self versus non-self. Specificity is the ability to react towards different antigens as non-self by age $2$ or $3$. Occasionally, cross-reactions occur, such as in syphilis where the bacteria share a hapten with human heart muscle cells.

Cellular Immunity and T Cell Subsets

Cellular immunity is mediated by T lymphocytes, which originate from hematopoietic stem cells (HSCs) and differentiate in the thymus. T cells express either $\gamma\delta$ or $\alpha\beta$ T-cell receptors (TCRs). The primary subdivisions of $\alpha\beta$ T cells are Helper T cells (Th), Cytotoxic T cells (Tc), and Regulatory suppressors (Treg).

Helper T cells (CD4+) are essential for activating B cells, Tc cells, and macrophages. Th cells are further divided into Th1 (produces $IFN-\gamma$ and $TNF\beta$ for cell-mediated immunity) and Th2 (produces $IL-4$, $IL-5$, $IL-6$, and $IL-10$ for IgG and IgE humoral responses). Other subsets include Th17, which recruits neutrophils, and Treg, which produces $IL-10$ and $TGF-\beta$ to suppress adaptive immune cells.

Cytotoxic T cells (CD8+) destroy foreign target cells, virus-infected cells, and tumor cells. They release granules containing perforin and granzymes. Perforin creates channels in the target cell membrane, while granzymes (such as Granzyme A and B) enter the cell to induce apoptosis (programmed cell death).

The Major Histocompatibility Complex (MHC)

MHC molecules present peptide fragments to TCRs. MHC Class I is expressed on all nucleated cells and is recognized by CD8+ Tc cells; the peptides presented typically come from the cytosol (endogenous antigens). MHC Class II is expressed only on specialized antigen-presenting cells (APCs) like dendritic cells and macrophages and is recognized by CD4+ Th cells; these peptides originate from vesicular spaces (external antigens).

T cell selection in the thymus involves positive selection (T cells that interact with MHC-peptide complexes survive) and negative selection (T cells that interact too strongly with self-peptides are destroyed).

Specialized Immune Phenomenon

Monoclonal antibodies are produced in labs by hybridomas—fused cells created from myeloma cells and sensitized lymphocytes. These hybridomas divide indefinitely, producing large quantities of a single specific antibody type used for treating cancer and diagnosing diseases like hepatitis.

Superantigens are proteins that activate significantly more T cells than normal by binding to conserved regions of MHC and TCR outside the normal binding site. This results in large-scale cytokine release and systemic inflammation.

Immunodeficiency results from defects in immune responses, which can be genetic (e.g., Severe Combined Immune Deficiency or SCID, where B and T cells do not form properly) or induced by microorganisms (e.g., AIDS caused by HIV, which infects CD4+ cells).

Hypersensitivity and Autoimmunity

Hypersensitivity is categorized into four types:

1. Type I (Immediate): Antibody-mediated (IgE) allergy or anaphylaxis (e.g., pollen, food). IgE binds to mast cells, and cross-linking by antigens releases histamine.
2. Type II (Cytotoxic): Antibodies react with antigens on cells, such as in mismatched blood transfusions.
3. Type III (Immune Complex): Large Ag-Ab complexes precipitate on blood vessel walls, causing tissue injury (e.g., SLE, Rheumatoid arthritis).
4. Type IV (Cell-mediated/Delayed): Delayed-type hypersensitivity (DTH) involving extensive tissue destruction by T cells (e.g., Multiple sclerosis).

Autoimmunity occurs when the immune system attacks self-antigens. Examples include Type I diabetes (pancreas), Myasthenia gravis (skeletal muscle), and Systemic Lupus Erythematosus (DNA and cellular constituents).