houman bio week 12 lecture 1

The Immune System: An Overview

Definition and Role

• The immune system is a collection of tissues, cells, and molecules that protect against foreign materials.

• It is not a single structure but rather a network of components distributed throughout the body.

• Immune cells travel in the bloodstream and migrate between tissues.

Major Functions

• Preventing microorganisms or pathogens from establishing an infection.

• Removing infections that have already been established.

• Removing old or dead cells.

• Involved in wound healing.

• Fighting mutated cells with the potential to become cancerous.

Pathogens

• Include viruses, bacteria, fungi, and parasites.

• For simplicity, these are collectively referred to as pathogens or microorganisms.

Immunity

• Immunity is the body's ability to recognize and eliminate disease (pathogens or tumor cells).

• It is the way the body builds resistance to disease through the immune system.

Divisions of the Immune System

Innate Immune System

• The first line of defense.

• Present from birth (hence, "innate").

• Blocks pathogens from entering cells and removes those that do.

Adaptive Immune System

• A specific response against particular pathogens.

• Involves antibodies produced by lymphocytes (white blood cells).

• Antibodies are specific to each pathogen.

Components of Innate and Adaptive Immune Systems

Physical and Chemical Barriers

• Prevent pathogens from entering the body.

Leukocytes (White Blood Cells)

Lymphoid Tissue

Physical and Chemical Barriers: Detailed Examples

Skin

• Made of epithelial cells, including keratinocytes that are packed closely together with tight junctions, making it difficult for pathogens to enter.

• The top layer is keratinized, consisting of dead cells filled with keratin.

  • Keratin repels water and is hard, hindering pathogen entry.

• Sebaceous glands secrete sebum around hair follicles, preventing bacterial growth.

  • Some bacteria have evolved to thrive on sebum, leading to conditions like acne.

Respiratory System

• Goblet cells produce mucus, which lines the airways.

• Dust and pathogens get trapped in the mucus.

• The mucus is then either spat out or swallowed.

• If swallowed, digestive enzymes in the stomach dissolve the mucus and its contents.

Stomach

• Highly acidic environment due to hydrochloric acid, which aids in digestion.

• Many pathogens cannot survive in the acidic conditions and are killed.

Cells of the Immune System: Leukocytes

Origin

• All blood cells, including leukocytes, are derived from hematopoietic stem cells in the bone marrow.

• Red bone marrow is the site of hematopoiesis (blood cell production).

Lineages

• Hematopoietic stem cells differentiate into two major lineages:

  • Myeloid cells

  • Lymphoid cells

Blood Smear

• A blood smear involves taking a drop of blood, smearing it on a slide, staining it, and observing it under a microscope.

• The majority of cells in a blood smear are red blood cells (erythrocytes).

• Leukocytes are larger cells with purple nuclei.

Leukocyte Function

• Leukocytes are found in the bloodstream and can migrate into tissues.

• Migration to tissues is essential for their function, especially in areas of infection.

• Leukocytes detect antigens (proteins or parts of proteins) that the immune system recognizes as foreign.

• Antigens can be components of pathogens or substances like pollen.

• Upon detecting an antigen, leukocytes induce an immune response.

Mechanisms of Action

• Phagocytosis: Some immune cells (phagocytes) engulf pathogens and release digestive enzymes to destroy them.

• Toxic Factors: Other cells release toxic factors that kill pathogens.

• Signaling Factors: Some cells release signaling factors that attract other immune cells to the area.

Granulocytes

• Characterized by a multi-lobed nucleus and cytoplasmic granules.

Neutrophils

• Most common leukocyte, making up 50-80% of leukocytes in the blood.

• The nucleus has a multi-lobed appearance.

• Primary role is to phagocytose foreign material, especially in bacterial infections.

• First cell type to arrive at sites of trauma or infection.

Eosinophils

• Rarer, making up 1-4% of leukocytes.

• Phagocytes, primarily targeting parasites.

• Release toxic substances that target parasites.

• Play a significant role in allergic reactions; production increases during allergic responses.

Basophils

• Least common, making up less than 1% of leukocytes.

• Similar to eosinophils in targeting parasites, but are non-phagocytic.

• Release toxins to kill cells.

• Play a role in allergic reactions, but to a lesser extent than eosinophils.

Monocytes

• Agranular cells (lacking granules in the cytoplasm).

• Have a kidney-shaped nucleus.

• Make up 2-8% of leukocytes in the blood. Production increases during infections, particularly viral and parasitic infections.

• Circulate in the blood and migrate into tissues, where they differentiate into macrophages.

Macrophages

• Mature form of monocytes found in tissues.

• Phagocytic cells that engulf and destroy pathogens.

• Can be circulating monocytes that migrate into tissue or tissue-resident macrophages that remain in the tissue.

• Act as surveillance mechanisms, looking out for potential pathogens.

• Macrophages are antigen-presenting cells, presenting antigens to other immune cells to initiate an immune response.

Dendritic Cells

• Characterized by cytoplasmic extensions resembling dendrites.

• Antigen-presenting cells.

• Mostly located in tissues, rarely seen in the blood unless migrating to lymphoid organs.

• Survey the area for potential pathogens.

Natural Killer (NK) Cells

• Contain many granules in their cytoplasm.

• Mount a nonspecific defense against pathogens.

• Kill cells infected by viruses or bacteria by releasing toxic substances from their cytoplasmic granules.

Mast Cells

• Have many cytoplasmic granules.

• Mostly found in tissues.

• Release histamine during allergic reactions, causing vasodilation of blood vessels.

• Vasodilation increases the leakiness of blood vessels, allowing other immune cells to exit the blood and enter the affected tissue more easily.