Immunology Simplified

                                              By Hatem Eleishi, MD
 

Introduction

      Immunology is one of the medical subjects that are renowned for little popularity among medical students and also among many physicians. However, knowledge of immunology is indispensable for an in-depth understanding of modern medicine and of diseases. Many of us read immunology books only to end up confused and lost in cytokine and mediator interactions that usually leave us wondering "which is which".
 

      Especially if you are one of those who have been struggling with immunology books and notes for some time but have not been able to get along, trying to convince you at the beginning of the course that immunology is a fascinating and easy subject or that the immune system is a neatly organized system might be extremely difficult. However, by the end of the first lecture/chapter of this course/book, you will already be hooked to continue and will think of immunology differently.

     It is important to note that this course is not about the minute details of cytokine effects and adhesion molecule interactions. Rather it is about the broad lines or the philosophy of the immune system if we may refer to it as a him. Once you acquire that level of knowledge, getting involved in any minute details and interactions will be like filling in the spaces.

     You will encounter many short stories and analogies in this course that might seem odd to a specialized immunologist at the start. However they have been included here for the purpose of simplification of the subject for beginners. For example, assuming that there was a primitive immune system many years ago that, under strong evolutionary pressure from invading microbes, evolved to a modern or specific immune system is only an oversimplification to help you understand the basics of immunology. Likewise are the dialogues between the immune system and his secretary and the stories about the social security numbers that the immune system made for the body cells. Once you get it and you start to speak immunology fluently, you will be able to easily sort out the fact from fiction in this course and will be able to read and understand any immunology article or sophisticated textbook.

Lecture / Chapter (1)

Why do we need an immune system?
 

We need an immune system to protect our bodies from microbes and from crazy cells.

Microbes:
Our environment contains a great variety of infectious microbes: viruses, bacteria, fungi, protozoa and multi-cellular parasites. They can infect man and cause tissue damage and disease. If they multiply unchecked, they will eventually kill their host. The immune system evolved to protect man from those pathogens and to eliminate them and to minimize the damage they cause.

Crazy cells:
Some cells in our bodies can become crazy and turn astray, that is, they no longer abide by the rules or obey the orders of our mediators, cytokines and hormones that instruct them when to proliferate and when to stop. Those cells are called malignant cells. Unchecked, they divide and proliferate at a high rate and, through one or more of several mechanisms, would eventually compromise organ/system function(s) and kill the host. The immune system also evolved to protect man from those crazy cells and to eliminate them and to minimize the damage they cause.

Who Do You Think Existed First, the Immune System or the Microbes?
 

Well….
If we say that the immune system evolved to combat microbes and cancer cells, then it goes without saying that the microbes and cancer cells probably existed first and their existence created the need for the creation of the immune system in order to combat them.


 

What is the Immune System Like? And How Does it Operate?

 

Since we agree that the immune system evolved to take action against microbes and cancer cells, then the best way to know about the immune system and the way it would operate is to know about microbes and cancer cells and their tactics of invading our bodies. If we do that, we can know or have a rather accurate expectation of how the immune system would/should be like.

 

So What Are Microbes Like?
 

Microbes are TWO TYPES and are TOO SMART, that's what microbes are like.
 

Two types: microbes are either extracellular or intracellular. Our bodies are made up of cells and extra-cellular tissue and matrix. Some microbes invade our bodies and live in between the cells in the extra-cellular tissues and matrix. Those are extracellular microbes. Other microbes, once they invade our bodies, target certain cells, gain access to their interior live and multiply inside them. Those are intracellular microbes.

 

Too smart: microbes are too smart in the sense that they are constantly changing and sharpening their tactics of body invasion.

The implications of those two simple facts, two types and too smart, are that:

1.The immune system has two types or lines of defense against microbes which are an extracellular line to combat extracellular microbes and an intracellular line to combat intracellular microbes.
 

2.The immune system, too, is too smart and is constantly changing and updating new tactics to counteract and prevent body invasion by microbes.


And that's what immunology is about..........…






The remainder of this lecture/chapter and of the entire course actually will be about the story of that constant struggle between microbes and immune systems.
 

But before we start our story, we will make small elaborations on "the two types of microbes" fact:
1.We will DIVIDE the extracellular microbes and ADD to the intracellular ones.
2.We will HIGHLIGHT an extracellular phase of existence for intracellular microbes.
 

DVIDE
 

We will divide the extracellular microbes into small extracellular microbes and large extracellular microbes. The index of size here is dictated by whether the microbe is small enough to be phagocytosed by a phagocytic cell or is too large to be phagocytosed by a phagocytic cell. Phagocytic cells are eater cells, cells that have the capacity to engulf/eat other cells or particles on condition of course that they are edible size wise, that is, they are small enough compared to the size of the phagocytic cell to be swallowed by it.

 

Thus,

Small extracellular microbes are microbes that can be phagocytosed by a phagocytic cell and include bacteria.


Large extracellular microbes are microbes that are too large that they cannot be phagocytosed by a phagocytic cell and include helminth parasites.


 

ADD
 

We will add to the intracellular microbes cancer cells as cancer cells operate at an intracellular level, specifically at the level of the nucleus, which is the same level at which typical intracellular microbes, viruses, operate. For that reason, from now on, when we refer to body defense lines against intracellular microbes and in particular viruses, it will be implied that this also refers to defense lines against cancer cells.


Thus,

Intracellular microbes are viruses, also some bacteria and, for simplification, cancer cells.


 

HIGHLIGHT

An Extracellular Phase of Presence for Intracellular Microbes:
 

Following their entry to the inside of our bodies and before their entry to the inside of their target cells, intracellular pathogens would typically temporarily be seen in the extracellular environment or space, the space where extracellular pathogens would normally be settling. Put differently, intracellular pathogens also have an extracellular presence or existence, albeit transient. During that phase, they are recognized and targeted also by our body's extracellular defense lines. This means that the extracellular defense systems, in addition to ridding our bodies of extracellular pathogens, also act against the intracellular pathogens during that extracellular phase when they are still targeting their final intracellular destinations. The implication of that fact is more responsibilities for the extracellular line of defense against pathogens and consequently an always more highly developed and more sophisticated extra-cellular defense line compared to the intracellular defense line.
 

The Story of the Constant Struggle between Microbes and Humans
 

Many many years ago, there existed a very primitive type of microbe world where extracellular pathogens (small as bacteria and large as helminth parasites) and intracellular pathogens (viruses and early cancers) conspired to invade early humans with a rather primitive set of microbial weapons.


At those early beginnings the immune system, too, had a primitive arsenal of weapons with which it succeeded to combat those pathogenic microbes most of the time.



We will now mention some of the primitive microbial weapons and how our early immune systems defended our bodies against them.
 

We will start with the small extracellular microbes (a), then the large extracellular microbes (b) and lastly the intracellular microbes (c).

 

The rest of this chapter will be posted soon