Session 14: Normal immune response

Session 14: Normal immune response

• Felician Sikujua (MD)

1

Back to top

Learning tasks

Learning tasks

At the end of this session, students are expected to be able to:

Explain types of immunity.

Identify cells of immune system.

Identify tissues of immune system.

Explain how cells of immune system respond to microbes.

2

Back to top

Introduction

Introduction

Immunity refers to protection against infections.

Immune system is the collection of cells and molecules that are responsible for defending the body against pathogenic microbes in the environment.

There are two (2) types of immunity

Innate immunity

Adaptive immunity

3

Back to top

Innate immunity (natural or native immunity)

Innate immunity (natural or native immunity)

It is mediated by cells and proteins that are always present and poised to fight against microbes, being called into action immediately in response to infection.

The innate immune response is able to prevent and control many infections.

However, many pathogenic microbes have evolved to overcome the early defenses, and protection against these infections requires the more specialized and powerful mechanisms of adaptive immunity.

4

Back to top

Components of innate immunity

Components of innate immunity

Epithelial barriers of the skin, gastrointestinal tract, and respiratory tract, which prevent microbe entry.

Phagocytic leukocytes (neutrophils and macrophages); and a specialized cell type called the natural killer (NK) cells.

Several circulating plasma proteins, the most important of which are the proteins of the complement system.

5

Back to top

Adaptive immunity

Adaptive immunity

Adaptive immunity is normally silent and responds (or “adapts”) to the presence of infectious microbes by becoming active, expanding, and generating potent mechanisms for neutralizing and eliminating the microbes.

Components of the adaptive immune system are: lymphocytes and their products.

6

Back to top

Types of Adaptive immunity

Types of Adaptive immunity

Humoral immunity.

Mediated by soluble proteins called antibodies that are produced by plasma cells [differentiated B lymphocytes (also called B cells)].

Antibodies provide protection against extracellular microbes in the blood, mucosal secretions, and tissues.

7

Back to top

Types of Adaptive immunity cont…

Types of Adaptive immunity cont…

Cell-mediated (or cellular) immunity.

Mediated by T lymphocytes (also called T cells).

T lymphocytes are important in defense against intracellular microbes.

They work by either

Directly killing infected cells (accomplished by cytotoxic T lymphocytes) or

Activating phagocytes to kill ingested microbes, via the production of soluble protein mediators called cytokines (made by helper T cells).

8

Back to top

Cells of Immune system

Cells of Immune system

The cells of the immune system consist of

Lymphocytes, which recognize antigens and mount adaptive immune responses.

Specialized antigen-presenting cells (APCs), which capture and display microbial and other antigens to the lymphocytes.

Various effector cells, whose function is to eliminate microbes and other antigens.

9

Back to top

Lymphocytes

Lymphocytes

Lymphocytes are present in the circulation and in various lymphoid organs.

Lymphocytes develop from precursors in the generative lymphoid organs.

There are two (2) types of lymphocytes

T lymphocytes are so called because they mature in the thymus.

B lymphocytes mature in the bone marrow.

10

Back to top

T lymphocytes

T lymphocytes

Effector cells of cellular immunity and the “helper cells” for antibody responses to protein antigens.

T cells do not detect free or circulating antigens, instead, they recognize only peptide fragments of protein antigens bound to proteins of the major histocompatibility complex (MHC).

This system ensures that T cells can recognize antigens displayed by other cells.

T cells function by interacting with other cells—either to kill infected cells or to activate phagocytes or B lymphocytes that have ingested protein antigens.

11

Back to top

B lymphocytes

B lymphocytes

Bone marrow–derived B lymphocytes are the cells that produce antibodies.

They are effector cells of humoral immunity.

B cells recognize antigen by means of membrane-bound antibody of the immunoglobulin M (IgM) class.

Unlike T cells, B cells can detect and respond to free or circulating antigens (chemical structures including soluble or cell-associated proteins, lipids, polysaccharides, nucleic acids, and small chemicals).

12

Back to top

Natural killer (NK) cells

Natural killer (NK) cells

Natural killer (NK) cells are lymphocytes that arise from the common lymphoid progenitor that gives rise to T and B lymphocytes.

However, NK cells are cells of innate immunity and do not express highly variable and clonally distributed receptors for antigens.

Therefore, they do not have specificities as diverse as do T cells or B cells.

NK cells kill cells that are infected by some microbes or are stressed and damaged beyond repair.

13

Back to top

Antigen presenting cells(APCs)

Antigen presenting cells(APCs)

Capture microbes and other antigens, transport them to lymphoid organs, and display them for recognition by lymphocytes.

The most efficient APCs are dendritic cells, which are located in epithelia and most tissues.

Others are macrophages and B cells.

14

Back to top

Effector cells

Effector cells

Leukocytes that perform the ultimate task of the immune response, which is to eliminate infections.

NK cells are front-line effector cells.

Antibody-secreting plasma cells are the effector cells of humoral immunity.

T lymphocytes (both CD4+ helper T cells and CD8+ CTLs) are effector cells of cell-mediated immunity.

Macrophages effectors cells for both types of immunity.

15

Back to top

Lymphoid tissue of immune system

Lymphoid tissue of immune system

Generative (primary) lymphoid organs.

In these organs lymphocytes express antigen receptors and mature.

These are thymus and bone marrow.

Peripheral (secondary) lymphoid organs.

In this organs adaptive immune responses develop.

These are lymph nodes, spleen, mucosal and cutaneous lymphoid tissues.

16

Back to top

Overview of normal immune responses

Overview of normal immune responses

The physiologic function of the immune system is defense against infectious microbes.

The early reaction to microbes is mediated by the mechanisms of innate immunity, which are ready to respond to microbes.

The defense reactions of adaptive immunity develop slowly, but are more potent and specialized.

17

Back to top

Overview of normal immune responses cont…

Overview of normal immune responses cont…

Microbes and other foreign antigens are captured by dendritic cells and transported to lymph nodes, where the antigens are recognized by naive lymphocytes.

The lymphocytes are activated to proliferate and differentiate into effector and memory cells.

Cell-mediated immunity is the reaction of T-lymphocytes, designed to combat cell-associated microbes (e.g., phagocytosed microbes and microbes in the cytoplasm of infected cells).

18

Back to top

Overview of normal immune responses cont…

Overview of normal immune responses cont…

Humoral immunity is mediated by antibodies and is effective against extracellular microbes (in the circulation and mucosal lumens).

Antibodies secreted by plasma cells neutralize microbes and block their infectivity, and promote the phagocytosis and destruction of pathogens.

19

Back to top

Overview of normal immune responses cont…

Overview of normal immune responses cont…

CD4+ helper T cells help B cells to make antibodies, activate macrophages to destroy ingested microbes, stimulate recruitment of leukocytes, and regulate all immune responses to protein antigens.

The functions of CD4+ T cells are mediated by secreted proteins called cytokines.

CD8+ (Cytotoxic T Lymphocytes) kill cells that express antigens in the cytoplasm that are seen as foreign (e.g., virus-infected and tumor cells).

20

Back to top

Key points

Key points

The physiologic function of the immune system is defense against infectious microbes.

Cells involved in innate immunity are neutrophils, macrophages and natural killer cells.

Humoral immunity is mediated by B cells while cell mediated immunity is mediated by T cells.

Antibodies are produced by differentiated B cells called plasma cells.

Dendritic cells are the most efficient antigen presenting cells.

Natural killer cells are front-line effector cells.

21

Back to top

Review questions

Review questions

Differentiate between innate immunity and adaptive immunity.

List three (3) components of innate immunity.

What is the role of antigen presenting cells?

List five (5) tissues of immune system.

Explain mechanisms on how T cells defend against microbes.

22

Back to top

References

References

Kumar V. ; Abbas A. K. ; Aster J. C.;(2013): Robbins and Contran Pathologic Basis of Disease (9th Ed.) Elsevier Saunders, USA. Pg. 99-109.

Mohan H.;(2010): Text book of Pathology (6th Ed.) Jaypee Brothers Medical Publishers, India. Pg. 61-64.

23

Back to top