Immunity to Infection 101: A superficial overview For the full story, signup for CBIO 4100/6100 Fall and Spring semesters

Why do you need a “course” in Immunology in order to understand parasitology? Because parasites invest a great deal of energy in avoiding the host immune response

Trypanosoma brucei

Trypanosoma cruzi

Crash course in Immunology: Major points: The immune system is a complex but understandable network of cells and secreted factors. This system is most easily understood by breaking it down into it component parts – keeping in mind that the parts all work together. The immune system has choices; an effective response to a pathogen requires that the immune system make the correct choices. Pathogens, including parasites, invest a great deal of energy in avoiding the host immune response.

Defining Characteristics of the Immune System Discriminates between self and non-self Essentially unlimited capacity to respond specifically to non-self Remembers non-self and responds more vigorously on subsequent encounters Immune system is probably evolved to deal with infections, to rid the body of infectious agents, and to prevent reinfection. Pathogens that it cannot get rid of are trouble.

Immune system: Components Innate: macrophages: phagocytosis complement-mediated lysis skin, mucosa: barrier NK cells: altered or missing MHC neutrophils, eosinophils, mast cells dendritic cells Adaptive: T cells and B cells dependent on innate to get activated but much more complex in part because of specificity often uses the innate as an end effector

Innate Immunity innate immunity is the initial response to microbes that prevents infections and in some cases eliminates pathogens the effector mechanisms of innate immunity are often used to eliminate microbes even in the adaptive response (innate often the first AND the end responders/effectors) innate immunity stimulates and directs adaptive responses

Epithelial Barriers e.g. gamma/delta T cells, NK-T cells

Exceptions – dead or dying cells

Features of Innate Immune Recognition components of the innate immune response recognize structures unique to microbes and required for their survival (molecular patterns; pathogen associated molecular patterns = PAMPs) these “pattern recognition receptors” (PRR) of innate system are germ-line encoded (therefore not variable)

Trinchieri and Sher Nature Reviews Immunology 7, 179–190 (March 2007) | doi: /nri2038

Phagocytosis

Effector functions of macrophages

Role of innate immune responses in defense 1. Local reaction to infection - inflammation - to bring effectors to the sites of infection 2. Systemic: endocrine actions of cytokines (e.g. fever, increased leukocyte production) note: both responses can be damaging Role of innate immune responses in stimulating adaptive responses activate and direct T cell and B cell response provide the “second signals” (pathogens which fail to activate innate responses generate poor acquired response)

Innate and adaptive responses to intracellular pathogens

Exceptions – dead or dying cells

Phases of an adaptive immune response Proliferation

Types of Adaptive Immunity

cellular and humoral adaptive immunity humoral immunity: initiated by antigen recognition by B cells, mediated by antibodies and effective against extracellular pathogens or their (toxic) products cell-mediated immunity: initiated by antigen recognition by T cells, mediated by T cells and other downstream effectors (e.g. macrophages), effective against intracellular pathogens (also important in transplant rejection and tumor immunity)

Effector functions of antibodies

Overview of Immune Responses in vivo

Effector function of antibodies Review points antibodies are produced by B cells and plasma cells and perform their effector functions at sites distant from their production - systemic prior antigen exposure results in the activation of long-lived plasma cells that continue to produce antibodies for many years, and memory cells which can be reactivated for increased production upon re-exposure.

cellular and humoral adaptive immunity humoral immunity: initiated by antigen recognition by B cells, mediated by antibodies and effective against extracellular pathogens or their (toxic) products cell-mediated immunity: initiated by antigen recognition by T cells, mediated by T cells and other downstream effectors (e.g. macrophages), effective against intracellular pathogens (also important in transplant rejection and tumor immunity)

Types of CMI: Different types of microbes elicit distinct T cell responses 1. T cell mediated macrophage activation (Th1 and CD8+ T cells): activation of phagocytes to kill vaculolarized microbes. Cytokine production recruits cells (monocytes) and activates them in the site of infection. Final effector is macrophage. DTH 2. Cytolytic T lymphocyte (CTL): lysis of cells with cytosolic pathogens: - final effector is the CTL itself. Purpose is to destroy cells harboring intracellular microbes - viruses, bacteria, protozoans. Cytokines production also important in many cases. 3. Th2 cells: response to helminth parasites: - especially mast/basophil and eosinophil mediated responses (allergy) 4. NK cell response - especially early in viral and other infections - role in destruction of infected cells prior to expansion of CTL. Also important in destruction of target cells which have down-regulated class I MHC.

Antigens must be “presented” to T cells

Pathways for Class II and Class I presentation of antigens

Immunity to Microbes Defense against microbes is mediated by innate and adaptive immunity Response is specialized for particular pathogens Survival of pathogens is dependent on immune evasion Tissue injury may result from response to microbe