Lecture 2 - Cell Types and Protein Components

What are the 5 myeloid cells (innate)?

Macrophage, dendritic cell, neutrophil, basophil, eosinophil

What are the 3 phagocytes?

Macrophage, dendritic cell, neutrophil

Innate cells: phagocytes

Function to surround, engulf, and break down microbes, small particles, and apoptotic host cells

Leukocytes =

Myeloid cells + Lymphoid cells

What are the 4 granulocytes?

Neutrophil, basophil, eosinophil, mast cell

Cytoplasmic granules

Pre-filled with inflammatory and mediators

Degranulation

1. Kills microbes in the area

2. Amplifies immune responses by recruiting more immune cells

What are the first cells on the scene of infection?

Neutrophils (most abundant)

Neutrophils (phagocyte and granulocyte)

• Rapidly migrate to the site of infection and inflammation

• Release their DNA as they die to trap bacteria to limit dissemination

Netosis

DNA forms sticky “nets”

Eosinophils (granulocyte)

Express cytoplasmic granules containing enzymes that are harmful to the cell walls of parasites

• High in parasitic worm infections and allergy

Basophils (granulocyte)

Involved in allergic responses and help fight parasitic worms

• Bind IgE, when IgE sees it’s antigen it causes histamine release

Mast cells (granulocyte)

Rapidly secrete pro inflammatory factors (histamine)

• Activated when IgE binds to antigen

Antigen

A substance (from a microbe or self) that stimulates an immune response

Antigen presenting cell (APC)

Capture antigens and process them for presentation to initiate the adaptive T cell responses

Antigen presentation

1. Phagocytosis of bacteria by APC (antigen presenting cell)

2. Digestion of microbial antigens

3. Presentation of antigen on MHC to T cell

Dendritic cells (phagocytes)

Most expert at antigen presentation

Macrophages (phagocytes)

Pretty good at antigen presentation

What are the 3 lymphoid cells?

T cells, B cells, NK cells

Each B and T cell cell has a unique surface receptor that responds to a:

Specific antigen

Natural Killer cells (lymphoid)

Lack a variable antigen receptor and function like innate cells

Antimicrobial peptides (AMPs)

Small proteins secreted by epithelial cells, phagocytes, and keratinocytes that are toxic to m icrboes

Antibodies

Secreted version of the B cell antigen receptor

• Do not directly kill, instead tag foreign antigens for removal by other immune cells or protein factors

Neutralization

Process where antibodies block pathogens

Opsononization

Process where antibodies (mainly IgG) coat the surface of pathogens, acting as "tags" (opsonins) that enhance recognition and destruction by phagocytes, such as macrophages and neutrophils

Complement activation

Process where C1q binds to antibody-antigen complexes (mainly IgM or IgG subclasses) on pathogens, initiating a protease cascade

Classical pathway

Antigen-antibody complexes

• Starts when IgG or IgM binds to pathogen

• These antibodies trigger an enzymatic cascade that cleaves C3

C3

Tags (opsonizes) pathogens to be eaten

• Activates C5, causing a membrane attack complex to form on the pathogen

C3b

Opsonizes pathogens

C3A (inflammation) and C5a

Recruit more immune cells

C5b

Initiates membrane attack complex that makes holes and kills microbes

Alternative (Spontaneous) Pathway

Antibody independent

• Spontaneous hydrolysis of C3

What are the 3 ways the complement system controls infection?

1. Amplifies inflammation

2. Increases phagocytosis

3. Directly kills pathogens

Cytokines

Responsible for cell to cell communication during an immune response

Chemokines

Direct traffic - draw immune cells towards sites of infection or inflammation