Understanding the Immune System in BIOS 101

Three lines of defense

The vertebrate body is defended from infection by three lines of defense: skin and mucous membranes, cellular counterattack, and specific immune response.

Skin

The first defense against invasion by microbes, consisting of an outer epidermis and a lower dermis.

Epidermis

The outer layer of skin, 10 to 30 cells thick, continuously shed and replaced.

Stratum corneum

The outer layer of the epidermis, where cells are continuously shed.

Basal layer

The layer of the epidermis below the stratum corneum, where cells are actively dividing.

Dermis

15 to 40 times thicker than the epidermis, providing structural support.

Subcutaneous layer

Layer beneath the dermis comprised of fat-rich cells that act as shock absorbers and provide insulation.

Chemical defense of skin

Includes oil glands that make the skin surface very acidic and sweat containing lysozyme, which digests the cell walls of bacteria.

Tears

Contain lysozyme to fight bacterial infections.

Digestive tract protection

Stomach acid and digestive enzymes provide protection, while sticky mucus traps microorganisms.

Cellular counterattack

The second line of defense involving cellular and chemical defenses that include cells and proteins that kill invading microbes.

Lymphatic system

The central location for the storage and distribution of substances involved in the second line of defense.

Macrophages

Cells that kill bacteria by ingesting them; a type of white blood cell.

Neutrophils

White blood cells that ingest bacteria and secrete chemicals to neutralize everything living in the infected area.

Natural Killer Cell

A type of white blood cell that attacks infected body cells and punctures their membranes with perforin.

Complement System

A system of ~20 proteins that circulate in the plasma in an inactive state until they encounter a fungal or bacterial cell wall.

Complement proteins

Proteins that aggregate to form a membrane attack complex that creates a pore in the foreign cell's membrane, causing water to burst the cell.

Interferons

Act as messengers to prevent a virus from spreading to new cells; the viruses still infect but cannot make new viruses in the cells that received the interferon message.

The Inflammatory Response

The infected/injured cell releases chemical alarm signals, such as histamine, increasing blood flow to the area, allowing phagocytes to migrate to the site of infection and attack the invaders.

Pus

The accumulation of dead phagocytes that form as a result of the inflammatory response.

The Temperature Response (Fever)

Bacteria that cause disease do not grow well at high temperatures; macrophages send a signal to the brain to raise the body's temperature above normal, producing a fever.

Specific Immunity

The third line of defense involving lymphocytes that are critical to the specific immune response.

Lymphocytes

White blood cells critical to the specific immune response, including T cell and B cell lymphocytes.

T cell lymphocytes

Lymphocytes that originate in the bone marrow but migrate to the thymus gland for maturation and recognize microorganisms and viruses by the chemical markers, or antigens, on their surfaces.

B cell lymphocytes

Lymphocytes that complete their maturation in the bone marrow and produce antibodies that coat antigens and mark cells for destruction.

Memory Cells

Cells produced by both B and T cells that recall previous exposure to an antigen and mount a quick attack against that antigen.

Macrophages

Cells that initiate the immune response by inspecting the surfaces of all cells they encounter.

Major Histocompatibility Proteins (MHC)

Special marker proteins carried by every cell in the body that attract T cells when complexed with antigens.

Antigens

Small pieces of foreign particles that remain after a foreign particle is taken in by cells and partially digested.

Cytotoxic T Cells

A type of T cell involved in directly killing infected cells.

B Cells: The Humoral Response

B cells respond to helper T cells, marking pathogens for destruction without directly attacking infected cells.

Antibodies

Markers produced by B cells that bind to antigens, allowing antigen particles to enter the B cell and marking cells for destruction.

Secondary Immune Response

A more effective immune response that occurs when a large group of lymphocytes that can recognize a pathogen remains after the first infection.

Helper T Cells

T cells that assist B cells in the immune response.

Phagocytes

Cells that migrate to the site of infection to attack invaders.

Infection by a Foreign Particle

The process where a foreign particle infects the body, is taken in by cells, and its antigens are moved to the cell surface.

Markers placed by B cells

Alert the immune system to kill pathogens.

Initial Immune Response

The delayed response to an antigen encountered for the first time.

T cell immune defense

The defense mechanism involving T cells that specialize in recognizing specific antigens.

Overactive Immune System

Many diseases reflect an overactive immune system.

Autoimmune disease

The body attacks its own tissues.

Examples of Autoimmune diseases

Multiple sclerosis, type 1 diabetes, rheumatoid arthritis, lupus, and Graves' disease.

Allergy

The body mounts a major defense against harmless antigens.

Asthma

A condition where the body reacts to harmless antigens, leading to breathing difficulties.

Vaccination

The introduction of a dead or disabled pathogen into a body.

How Vaccination Works

Vaccination triggers an immune response against the pathogen, without an infection occurring.

First Vaccine

The first vaccine used cowpox to prevent smallpox.

Flu Vaccine Design

The flu changes quickly so new vaccines are always needed.

Antigen Makeup Change

Some viruses change their antigen makeup and prevent detection even after a vaccination.

Flu Viral Genes

Flu viral genes that code for surface proteins mutate quickly.

mRNA Vaccines

A new type of vaccines that use messenger RNA to instruct cells to produce a protein that triggers an immune response.

Primary Response

The initial immune response to an antigen.

Secondary Response

The immune response upon re-exposure to an antigen, which is typically faster and stronger.

SARS Antigen Impact

Prior exposure to the SARS antigen impacts the speed or magnitude of the primary response to the COVID-19 antigen.

Lines of Defense

The body has multiple lines of defense to protect us from pathogens.

Cells of the Immune System

The immune system contains many cells that are able to detect, attack, and generate antibodies to foreign invaders.

Antibodies

Proteins that mark invaders so that the immune system knows they need to attack.

Memory of Invaders

Once you have met an invader, the immune system keeps a record so they can make antibodies more quickly the next time.

Function of Helper T Cell

Commander of the immune response; detects infection and sounds the alarm, initiating both T cell and B cell responses.

Memory T Cell

Provides a quick and effective response to an antigen previously encountered by the body.

Cytotoxic T Cell

Detects and kills infected body cells; recruited by helper T cells.

Suppressor T Cell

Dampens the activity of T and B cells, scaling back the defense after the infection has been checked.

B Cell

Precursor of plasma and memory cells; specialized to recognize specific foreign antigens.

Plasma Cell

Biochemical factory devoted to the production of antibodies directed against specific foreign antigens.

Mast Cell

Initiator of the inflammatory response, which aids the arrival of leukocytes at a site of infection; secretes histamine and is important in allergic responses.

Monocyte

Precursor of macrophage.

Macrophage

The body's first cellular line of defense; also serves as antigen-presenting cell to B and T cells and engulfs antibody-covered cells.

Natural Killer Cell

Recognizes and kills infected body cells; detects and kills cells infected by a broad range of invaders.