IHD 2

Non-Specific Defense Mechanisms

  • Prevent the spread of infection until specific mechanisms activate (3-4 days).

  • Once activated, lymphocytes are very effective.

Cell-Mediated Immunity

  • Involves T-lymphocytes, natural killer cells (Innate), and macrophages (Innate).

Antibody-Mediated Immunity (Humoral Mediated)

  • Specific defense mechanism can be direct or activate complement system.

Cell-Mediated Defense Mechanisms

Lymphocytes

  • Two primary types: T-lymphocytes and B-lymphocytes, derived from lymphoblasts in red bone marrow.

  • Migrate to lymphatic tissues and organs to gain immunological competency.

T-Lymphocyte Development

  • Immature T-lymphocytes (lymphoid progenitor cells) migrate to thymus gland.

  • Selection process removes non-responsive T-cells (over 95% fail).

  • Mature T-cells stored in inactive state as small lymphocytes.

T-Cell Receptors (TCR)

  • Specific molecules on T-cell surface recognizing antigens.

  • Unique TCR produced through gene rearrangement leads to millions of potential antigen recognition.

Activation Process

  • T-cells activate by binding TCR with MHC-presented antigens on antigen-presenting cells (APCs).

Antigen-Presenting Cells (APCs)

  • Include macrophages, neutrophils, and dendritic cells.

  • Phagocytose antigens, present fragments linked with MHC to T-cells.

  • Only <1 in 10,000 cells may respond.

Sensitized T-Cell Pathway

  1. TH cell activated by antigen–MHC complex presented by APC.

  2. Activated TH cell enlarges and divides (clonal expansion).

  3. Differentiation into T-cell subsets: Cytotoxic T-cells (Tc), Helper T-cells (Th), Suppressor T-cells, Memory cells.

T-Cell Subsets

  • Cytotoxic T-lymphocytes (CD8+): Destroy infected cells and tumor cells.

  • Helper T-lymphocytes (CD4+): Enhance immune response, stimulate B-cells.

  • Suppressor T-lymphocytes (CD8+): Potentially turn off immune response.

  • Memory cells (CD4+ or CD8+): Remain for years, prompt secondary response.

T-Lymphocyte Activation Steps

  1. T-cells migrate to infection site, releasing cytokines and cytotoxins.

  2. Cytotoxic T-cells kill target cells using molecules like perforin-1.

  3. Helper T-cells activate antibody production by B-cells.

Antibody-Mediated Defense Mechanisms

B-Lymphocyte Development

  • Develop in bone marrow from hematopoietic stem cells (HSCs).

  • Move to lymphoid tissues such as lymph nodes and spleen.

B-Cell Activation and Differentiation

  1. APC presents foreign antigens; B-cells binds independent antigen recognition but usually need TH cell stimulation.

  2. Activation via TH cell recognition of antigen–MHC complex on B-cell.

  3. Activated B-cells undergo clonal expansion to form plasma and memory cells.

Immunoglobulin Structure

  • Immunoglobulins (antibodies) consist of 4 polypeptide chains (2 heavy, 2 light).

  • C-region (Fc) interacts with immune cells; V-region binds specific antigens.

Immunoglobulin Classes

  • IgG: 75% of plasma immunoglobulins; binds to phagocytes, effective against bacteria and viruses.

  • IgM: Primary response; pentamer; effective in agglutination and complement activation.

  • IgA: Found in body secretions; effective against viral/bacterial attachment.

  • IgD: Rare; found on B-cells; involved in binding to basophils.

  • IgE: Bound to mast cells; involved in allergic responses and defense against parasites.

Vaccine Mechanisms

  • Vaccination exposes immune system to antigens to generate memory cells/antibodies.

  • Antigens used may include killed or weakened pathogens, non-pathogenic strains, or altered bacterial toxins.

Primary and Secondary Response

  • First encounter with a new pathogen elicits a primary response (3-14 days).

  • Subsequent exposure triggers a rapid, powerful secondary response due to memory cells, often preventing disease reestablishment.