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Ch8 Immune System

Copyright © 2025 Mark Song

Immune system is a distributed set of cells in blood, lymph fluid, tissues, and organs

It can distinguish self and non-self

protect against infections

isolate and remove foreign substances

remove cancer cells

Leukocytes, AKA WBC

Cell-cell interactions and communications

Innate response

defend without distinguish identities

Self vs non-self

via Pattern Recognition Receptors (PRR) recognize if it is a pathogen

Own cell express protein that inactivate complement system (against C3b by ComplementReceptor1 etc.)

NK cells use a combination of activating (self protein) and inhibitory receptors (non-self or stress protein) which the two protein will occur in a specific balance

  1. Physical Barriers

    e.g., skins, hair, mucus, stomach acid, antimicrobial chemicals

  1. Phagocytes

    Cells that engulf pathogens and cellular debris (phagocytosis) and merge with lysosomes

    Including Neutrophils (chemotaxis) and Macrophages (PRRs and complement system)

  1. Natural Killer cells

    Recognition and destruction of cancer cells/virally infected cells

  1. Complement system

    Circulating protein in the plasma that assist destruction of pathogens

  1. Inflammation

    A local response to infection and injury that destroy potential pathogens

Example: response to a splinter

  1. cells, including epithelial and leukocytes senses tissue damages and releases signal molecules, that increase capillary permeability and dilate local arterioles causing edema (swelling)
  1. Neutrophils (and other phagocytes) moves out of blood to the inflamed area, which is a multistage movement and is a “chemotaxis” (directed migration of a cell in response to a chemical signal)
  1. Killings of pathogens via engulfment by phagocytes and complement system

    Complement system effects:

    C3b interacts with pathogen membrane and marks it for destruction via phagocytosis or activation of MAC

Adaptive immunity

depends on recognizing specific pathogens

Lymphocytes, cells in adaptive immune response system, from bone marrow and found in the lymphatic system and blood

B Cells (plasma cells, memory B cells)

Helper T Cells

Cytotoxic T Cells

Antigens

any kind of markers that the immune system can recognize

Proteins or sugars on the outside of viruses

Immune system read antigens with specificity

exist on viruses, bacteria, allergens, parasites, proteins, tumor cells, normal cells

B Cells

each express one antigen receptors

the receptor is immunoglobulin, which contains multisubunit protein

When antigen binds, B cell undergoes division (clonal expansion) which give rise to antibody-secreting plasma cells and memory B cells

The stem (Fc) portion of antibody interacts with receptors on phagocytes and stimulating phagocytosis and can also activate complement system

Helper T cells

Binds to antigen complexed with MHC Class II molecules made by some cells (including Macrophages and B cells)

It also require co-stimuli to fully activates

Secret signals that further activate themself and paracrine effects to B cells, Cytotoxic T cells, and NK cells

Cytotoxic T cells

Activated with MHC Class I (expressed by all cells), requires signal from helper T cells for activation

Cytotoxic T cells are activated by antigen complexed with MHC Class I (except red blood cell, present virus to the outside) that target cancer and virally infected cells, and required activation by helper T cell. This will lead to cells apoptosis

After an infection

Most activated lymphocytes undergo apoptosis

Antibodies can last for days-months

Memory B cells remains

Some helper T and cytotoxic T remains as memory cells

The first antigen exposure would have small amount of specific antibody (7-10 days after infection), but the second would have significantly more specific antibody (2-5 days after infection)

Roles of adaptive immunity

Encounter and antigen recognition

Lymphocytes activation

Coordinate attack

Memory

Active vs Passive Immunity of Adaptive immunity

Active: resistance due to exposure to pathogens of vaccinations
Passive: transfer of antibodies from gestation, breast feeding, therapy

Central tolerance, during early development, B and T cells with high affinity to self antigen go through apoptosis, which is in a process called Clonal Deletion

Peripheral tolerance, regulatory T cells, modulate immune response which prevent attacking self

Regulatory T cells, recognizes MHC class II and then signal to autoreactive B-cell, which would cause apoptosis (for example)

Autoimmune disease

Immunity attack cell, e.g., Multiple sclerosis (myelin in CNS), type 1 diabetes mellitus (beta islet cells in the pancreas)

Causes

  1. Failure to clonal deletion of lymphocytes
  1. Problem with regulatory T cells
  1. Exposure to pathogens that contains antigens similar to out own protein can lead to cross-reactivity

Innate immune responses

Rapid, no specificity, no memory

Adaptive immune responses

Slower, specificity, with immunological memory

Interaction between Innate and Adaptive immune responses

Antibodies can recruit complement (C1b) system and phagocytes

Inflammation recruits lymphocytes

Macrophage present MHC class II to helper T cells

Helper T cells increase activity of natural killer cells