Lecture 17 & 18 (4/14/26 & 4/21/26): Immune System

Why do new strains of the flu arise?

Genes of the flu virus can mutate, and versions of genes can be swapped within the host cells

pathogen

organisms and other agents that cause disease.

flu (reassortment)

the process where two different influenza viruses infect a single cell and swap genetic segments, creating a new hybrid strain

epidemic

a sudden increase in the number of disease cases above what is normally expected in a specific, localized population or region.

pandemic

an epidemic that has spread over several countries or continents, usually affecting a large number of people.

vaccine

• stimulate the immune system (adaptive branch) to make memory cells specific to a particular pathogen, so the immune system has the tools to combat a particular infection

• do not usually prevent a person from being infected

innate immune system

operate without previous exposure to a pathogen

external innate defenses

Skin, secretions, mucous membranes

internal innate defenses

phagocytic cells, natural killer cells, defensive proteins, inflammatory response

adaptive immune system

activated by exposure to specific pathogens

ex: antibodies, lymphocytes (B cells and T cells)

lymphatic system/lymph node

involved in internal innate defenses and adaptive defenses

How does the adaptive immune system differ from the innate immune system?

Only the adaptive has memory of a previous infection, and has the ability to target a specific pathogen

antigen

molecules that can trigger an immune response (pieces of the invader, not whole cell), and can destroy antibodies

Components of the adaptive immune system:

Lymphocyte origin:

Lymphocyte maturation:

Lymphocyte activation:

Lymphocyte transport:

Lymphocyte origin: bone marrow

Lymphocyte maturation: Bone marrow (B cells), Thymus (T cells)

Lymphocyte activation: Spleen, Lymph nodes

Lymphocyte transport: Lymphatic ducts, blood vessels

What is a major advantage of adaptive immunity?

it enables a rapid defense against an antigen that has been previously encountered

Specificity

particular lymphocytes recognize only particular pathogens or their pieces

Immunological memory

primary & secondary immune response

antibody-mediated (humoral) immune response (AMI)

• antibodies released from B cells attack pathogens in blood, lymph & tissue fluids - not in cells (free floating things)

• B cells bind to antigens during the activation phase

cell-mediated immune response (CMI)

• cytotoxic T cells attack pathogen-infected cells or cancer cells

• T cells bind only to antigens displayed by other body cells during the activation phase

phagocytic cell

Crucial for CMI & AMI response, where they engulf many different pathogens/invaders and present foreign antigens that stimulate adaptive immune responses which activate helper T cells

cytokine

small signaling proteins secreted by immune and non-immune cells that act as mediators of inflammation, immune responses, and cell communication

helper T cell

coordinate adaptive response by stimulating both humoral and cell-mediated immunity by releasing cytokines

cytotoxic T cell

fight body cells infected by a virus, and have antigen receptors that bind to antigens presented on the surfaces of host cells

B cell

produce multiple antibodies

antibody

Y-shaped proteins produced by B-cells to identify and neutralize foreign invaders like bacteria and viruses

What can produce memory cells?

Helper T cells, Cytotoxic T cells, B cells

memory response

the immune system's ability to rapidly recognize and destroy pathogens it has previously encountered, resulting in a faster, stronger, and longer-lasting response

memory cells

Memory T cells and B cells confer future immunity to a certain virus

autoimmune disorder

immune reaxction against some of the body’s own cells

• anti-self lymphocytes —> diabetes type 1, systemic lupus, multiple sclerosis

• treatment (generally no cure): replacement therapy, immune suppression

What happens if immune system cells do not distinguish self from non-self?

autoimmune disorder