Immune System: Study Notes

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Immune System

The ability of the host to fight the disease causing organisms, conferred by the immune system is called immunity.

Innate Immune System

• The response is non-specific.
• Exposure leads to an immediate maximal response.
• Includes both cell-mediated and humoral components.
• No immunological memory.
• Found in nearly all forms of life.

Adaptive Immune System

• Pathogen and antigen-specific response.
• There is a lag time between exposure and the maximal response.
• Includes both cell-mediated and humoral components.
• Exposure leads to immunological memory.
• Found only in jawed vertebrates.

Immune Cell Types:

• Mast cells: These cells dilate blood vessels and induce inflammation through histamine release. They recruit macrophages and neutrophils and are involved in wound healing and allergic responses. Found in connective tissues and mucous membranes.
• Macrophages: Phagocytic cells that consume foreign pathogens and cancer cells. They also stimulate other immune cells. Macrophages migrate from blood vessels into tissues.
• Natural killer cells: These cells kill tumor cells and virus-infected cells. They circulate in the blood and migrate into tissues.
• Dendritic cells: These cells present antigens on their surface, triggering adaptive immunity. They are present in epithelial tissue, including the skin, lungs, and digestive tract, and migrate to lymph nodes upon activation.
• Monocytes: These cells differentiate into macrophages and dendritic cells in response to inflammation. They are stored in the spleen and move through blood vessels to infected tissues.
• Neutrophils: The first responders at sites of infection or trauma. Neutrophils are the most abundant phagocytic cells, making up 50-60% of all leukocytes. They kill pathogens by releasing toxins and also recruit other immune cells. They migrate from blood vessels into tissues.
• Basophils: These cells defend against parasites. They release histamines that cause inflammation and may contribute to allergic reactions. Basophils circulate in the blood and migrate into tissues.
• Eosinophils: These cells release toxins that kill bacteria and parasites but can also cause tissue damage. They circulate in the blood and migrate to tissues.

ADAPTIVE IMMUNE SYSTEM:

• The adaptive immune system consists mainly of two types of lymphocytes, B-cells and T-cells. B-cells govern the humoral response, while T-cells mount the cell-mediated response.
• All cells of the immune system are created in the bone marrow, but B- and T-cells mature in different locations.
• B-cells mature in the bone marrow (although the B in their name originally stood for the bursa of Fabricius, an organ found in birds), and T-cells mature in the thymus.

Humoral Immunity

• Humoral immunity, which involves the production of antibodies.
• An antigen is a substance that is foreign to the body and stimulates an immune response.
• An antibody is a glycoprotein (immunoglobulin) made by plasma cells derived from B-lymphocytes, secreted in response to an antigen; the variable region of the antibody molecule is complementary in shape to its specific antigen.
• Antibodies target a particular antigen. They contain two heavy chains and two light chains.
• They have a constant region and a variable region; the tip of the variable region is the antigen-binding region.
• When activated, the antigen-binding region undergoes hypermutation to improve thespecificity of the antibody produced. Cells may be given signals to switch isotypes of antibody (IgM, IgD, IgG, IgE, IgA).
• Circulating antibodies can opsonize pathogens (mark them for destruction), cause agglutination (clumping) into insoluble complexes that are ingested by phagocytes, or neutralize pathogens.
• Cell-surface antibodies can activate immune cells or mediate allergic reactions.
• Memory B-cells lie in wait for a second exposure to a pathogen and can then mount a more rapid and vigorous immune response (secondary response).

Cell Mediated Immunity

• Cell-mediated (cytotoxic) immunity is centered on the functions of T-cells.
• The peptide hormone thymosin promotes T-cell development.
• Helper T-cells (Th or CD4 ) respond to antigen on MHC-II and coordinate the rest of the immune system, secreting lymphokines that are capable of recruiting other immune cells(such as plasma cells, cytotoxic T-cells, and macrophages) and increasing their activity. Because MHC-II presents exogenous antigens, CD4 T-cells are most effective against bacterial, fungal, and parasitic infections.
  • Th1 cells secrete interferon gamma, which activates macrophages.
  • Th2 cells activate B-cells.
• Cytotoxic T-cells (Tc , CTL, or CD8 ) respond to antigen on MHC-I and kill virally infected cells by injecting toxic chemicals that promote apoptosis into the infected cell. CD8 T-cells respond to antigens presented on MHC-I molecules.
• Suppressor (regulatory) T-cells tone down the immune response after an infection and promote self-tolerance.
• Memory T-cells serve a similar function to memory B-cells.

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